Construction of a Rapid Feather-Degrading Bacterium by

Dec 16, 2015 - Keratinase is essential to degrade the main feather component, keratin, and is of importance for wide industrial applications. In this ...
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Construction of a Rapid Feather-Degrading Bacterium by Overexpression of a Highly Efficient Alkaline Keratinase in its Parent Strain Bacillus amyloliquefaciens K11 Lian Yang, hui wang, Yi Lv, Yingguo Bai, Huiying Luo, Pengjun Shi, Huoqing Huang, and Bin Yao J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b04747 • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 17, 2015

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Journal of Agricultural and Food Chemistry 1

Construction of a Rapid Feather-Degrading Bacterium by Overexpression of a Highly Efficient Alkaline Keratinase in its Parent Strain Bacillus amyloliquefaciens K11 Lian Yang, Hui Wang, Yi Lv, Yingguo Bai, Huiying Luo, Pengjun Shi, Huoqing Huang*, Bin Yao*

Key Laboratory for Feed Biotechnology of the Ministry of Agriculture, Feed Research Institute, Chinese Academy of Agricultural Sciences, Beijing 100081, P. R. China

Running title: An extreme alkaline keratinase from B. amyloliquefaciens.

* Corresponding authors. Key Laboratory for Feed Biotechnology of the Ministry of Agriculture, Feed Research Institute, Chinese Academy of Agricultural Sciences, No. 12 Zhongguancun South Street, Beijing 100081, P. R. China. Tel.: +86 10 82106053; fax: +86 10 82106054. E-mail addresses: [email protected] (B. Yao), [email protected] (H. Huang). 1

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ABSTRACT: Keratinase is essential to degrade the main feather component, keratin, and is

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of importance for wide industrial applications. In this study, Bacillus amyloliquefaciens strain

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K11 was found to have significant feather-degrading capacity (completely degraded whole

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feathers within 24 h). The keratinase encoding gene, kerK, was expressed in the Bacillus

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subtilis SCK6. The purified recombinant KerK showed optimal activity at 50 °C and pH 11.0

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and degraded whole feathers within 0.5 h in the presence of DTT. The recombinant plasmids

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harboring kerK were extracted from B. subtilis SCK6 and transformed into B.

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amyloliquefaciens K11. As results, the recombinant B. amyloliquefaciens K11 exhibited

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enhanced feather-degrading capacity with shortened reaction time within 12 h and increased

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keratinolytic activity (1500 U/ml) by 6-fold. This efficient and rapid feather-degrading

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character makes the recombinant strain of B. amyloliquefaciens K11 potential for applications

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in feather meal preparation and waste feather disposal.

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Keywords: Bacillus amyloliquefaciens K11, Keratinase, Extreme alkaline, Gene expression

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INTRODUCTION

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Keratin is a family of insoluble structural proteins that represent the major component of

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mammalian hair, nails, wool, hoof and horn, poultry feathers, and so on.1,2 Keratinous wastes

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constitute a serious environmental contaminant that are mainly from poultry and leather

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industries, for instance, large quantities of feathers, approximately 8.5 million tons annually,

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are produced worldwide as a by-product of chicken poultry.3 The feathers are composed of

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over 90 % of keratin protein that mainly consists of small and essential amino acid residues

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such as glycine, valine, serine and cysteine. These and other residues are cross-linked by

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disulfide bonds, hydrogen bonds and hydrophobic bonds and tightly packed into a super

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coiled polypeptide, forming an insoluble, highly stable structure.1,4 The mechanical stability

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makes keratin highly resistant to proteolytic degradation of trypsin, pepsin and papain.5,6 As

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results, degradation of waste feathers is very slow in nature and causes serious environmental

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pollution.7 How to dispose waste feathers has been a major concern of poultry industry. A

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current value-added approach is to convert feathers into digestible dietary protein, i.e. feather

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meal, by using conventional physical and chemical treatments.8 However, this process not

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only destroys keratin amino acids and decreases protein quality, but also consumes large

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amounts of energy. 9

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Alternatively, microbial degradation or enzymolysis of feathers is becoming an attractive

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approach to manage keratinous wastes.9-11 To date, a number of keratinolytic microorganisms

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have been reported, including some species of Bacillus, Actinomycetes and fungi.12−16 These

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organisms can produce proteases specific for insoluble keratin substrates, i.e. keratinase.10

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Processing waste feathers by these keratinolytic microorganisms and related enzymes

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represents an alternative method, as it offers a specific, cost-effective, and environmentally

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benign solution to produce valuable products.9,17

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Keratinase has application potentials in many fields. In feed industry, poultry feathers are

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degraded by keratinase into feather meal, which supplementation into animal diets can

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provide essential amino acids and replace soybean meal at 7 % dietary level.18 Besides,

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keratinolytic enzymes might be used in agriculture, pharmaceutical, biomedical fields and

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leather industry. KERUS from Brevibacillus brevis US575, has been reported, could

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accomplish the whole process of dehairing by oneself.19 Moreover keratinase also can be

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applied in the cosmetics industry for high purity. Currently, there are only a few commercial

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keratinases from Bacillus licheniformis PWD-1 under the trade names Versazyme, Valkerase,

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Prionzyme and PURE100. Due to the slow feather-degrading efficiency and high production

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cost, keratinase, unlike other feed enzymes (phytases, xylanase, mannanase, etc), is not

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applied widely. Thus it is of importance to obtain a super feather-degrading microorganism

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and achieve low-cost production of highly active keratinase for large-scale industrial

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processes.

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Bacillus subtilis is a highly efficient system that has been used for the production of

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heterologous keratinase.20−22 Its non-pathogenic characteristic and free of endotoxins make it

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earn a “generally recognized as safe” (GRAS) status by the American Food and Drug

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Administration (FDA). The recombinant B. subtilis system not only secretes extracellular

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proteins directly into the culture medium and simplifies the downstream processing procedure,

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but also has non-biased codon usage.23,24

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In this article, a highly efficient feather-degrading bacterium, Bacillus amyloliquefaciens

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K11, was reported, which was able to disintegrate chicken feathers completely within 24 h.

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By cloning and heterologous expression of the coding gene, kerK, the keratinase was

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produced in B. subtilis SCK6, and exhibited substantial keratinase activity. The recombinant

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plasmid pUB110-kerK was then extracted from B. subtilis SCK6 and transformed its parent

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strain. As results, the feather-degrading ability of B. amyloliquefaciens K11 was enhanced

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significantly, shortening the feather disintegration duration to 12 h and increasing the

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keratinase production by 6-fold as compared with that of strain K11.

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MATERIALS AND METHODS

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Strains, Plasmids, Materials and Chemicals. B. amyloliquefaciens K11 with high

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neutral protease-producing capacity was isolated and deposited at Agricultural Culture

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Collection of China, Beijing under the Registration No. ACCC19735. B. subtilis SCK6

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(BGSC 1A976) and plasmid pUB110 were gifts from Dr. Daniel Zeiglerat of the BGSC. The

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pGEM-T Easy vector was purchased from TransGen (Beijing, China). Chicken feathers were

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collected from a farm in Beijing suburb, washed with tap water, soaked in 70 % ethanol for 1

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h, and air-dried.25 Folin-Ciocalteu’s phenol reagent, casein, and other chemicals were of

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analytical grade and commercially available.

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Feather-degrading Capacity Assessment and Isolation of the Native Keratinase. B.

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amyloliquefaciens K11 grown in 20 ml of LB medium at 37°C for 12 h at the agitation speed

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of 200 rpm was used as seed culture. 1% of the seed culture was inoculated into 150 ml of

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fresh feather medium containing (per litre) 0.5 g of NaCl, 0.4 g of KH2PO4, 0.3 g of K2HPO4,

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and 4 g of feather (the sole source of carbon and nitrogen) at 37 °C. The feather degrading

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degree was visualized at different time intervals. After 36-h incubation, the culture broth was

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centrifuged at 4 °C and 12,000 ×g for 10 min, and the supernatant was concentrated by a

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Vivaflow 200 membrane of 3-kDa molecular weight cutoff (Vivascience, Hannover,

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Germany). The crude enzyme was analyzed by sodium dodecyl sulfate-polyacrylamide gel

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electrophoresis (SDS-PAGE) according to the method of King and Laemmli (1971).26 The

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most obvious band in the SDS-PAGE gel was excised and identified using liquid

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chromatography-electrospray tandem mass spectrometry (LC-ESI-MS/MS) by Tianjin

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Biochip Co. Ltd. (Tianjin, China).

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Cloning

of

the

Keratinase-encoding

Gene

(kerK).

Genomic

DNA of

B.

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amyloliquefaciens K11 was extracted using TIANprep Midi Bacteria DNA kit (TIANGEN,

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Beijing, China). The primer pair kerKF and kerKR (Table 1) was designed according to the

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sequence alignment of the peptide fragments from LC-ESI-MS/MS and the putative

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keratinase gene of B. amyloliquefaciens Y2 (Accession No. CP003332.1). The keratinase

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gene of B. amyloliquefaciens K11, designated kerK, was amplified with an annealing

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temperature of 55 °C, and the PCR product was purified and sequenced.

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Sequence Analysis. The sequence assembly was carried out using the DNAMAN6.0

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software. Homology analysis of the nucleotide and amino acid sequences was performed

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using the BLAST program available from the National Center for Biotechnology Information.

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The putative signal peptide was predicted online (http://www.cbs.dtu.dk/services/SignalP/).

106 107

Construction of the Expression Vector pUB110-kerK. The expression vector

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pUB110-kerK was constructed by using the simple cloning method.27 Briefly, the linear

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backbone of plasmid pUB110 without the mob gene and the gene fragment including the

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native promoter, kerK and the terminal sequence of B. amyloliquefaciens K11 were amplified

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by using the primers pUB110F and pUB110R and kerKF and kerKR (Table 1), respectively.

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The PCR products were gel purified and used as templates for the prolonged overlap

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extension (POE)-PCR using the high fidelity KOD-Plus-Neo enzyme (TOYOBO, Osaka,

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Japan; KOD-401) without primers. The POE-PCR conditions were denaturation at 95 °C for 1

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min; 30 cycles of 95 °C denaturation for 20 s, 60 °C annealing for 40 s, and 72 °C extension

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for 3 min; followed by 72 °C extension for 10 min.

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Transformation of pUB110-kerK into B. subtilis SCK6. The POE-PCR product

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multimers (5 µl) were added to 100 µl of freshly prepared B. subtilis SCK6 competent cells,27

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and agitated at 37 °C and 200 rpm for 90 min. The cultures were then incubated at 37 °C

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overnight, followed by cell plating on LB plates with kanamycin (20 µg/ml). Colonies were

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then transferred to LB medium for 12-h growth at 37 °C and 200 rpm. Recombinant plasmids

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were then extracted from B. subtilis SCK6 and were verified by restriction digest and

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sequencing.

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Functional Characterization of kerK Gene in B. subtilis SCK6. To determine the

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function of kerK, colonies of recombinant B. subtilis SCK6 containing pUB100-kerK were

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grown on LB plates containing 2 % skimmed milk and in the feather medium at 37 °C for 24

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h, respectively. Cells of the recombinant B. subtilis SCK6 harboring the empty vector pUB110

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were cultured under the same conditions as controls. The feather-degrading capacity and

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keratinase activity were assessed as described above and below, respectively.

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Purification of Recombinant KerK. Recombinant B. subtilis SCK6 harboring

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pUB100-kerK was grown at 37 °C for 36 h in the feather medium, and the culture supernatant

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was collected by centrifugation at 12,000 ×g for 10 min to remove cell debris. The culture

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supernatant was concentrated by a hollow fiber (cutoff 3 kDa; Motianmo, Tianjin, China)

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followed by vacuum freeze-drying. The crude enzyme was precipitated by solid ammonium

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sulfate up to 85 % saturation level, stayed intact for 12 h, and centrifuged at 8000 ×g for 10

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min. The pellet was then dissolved in 50 mM Tris-HCl (pH 8.0) and dialyzed overnight

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against the same buffer at 4 °C.

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The SDS-PAGE analysis was performed according to the method of King and Laemmli

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(1971)26 with some modifications. Briefly, before enzyme loading, 10 µl extra 20 % SDS was

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added, and then was mixed with the loading buffer. The mixture was boiled for approximately

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10 min.

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Enzyme Activity Assay. Keratinolytic activity was assayed following Vermelho et al.

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(2009)28 with some modifications. Briefly, feather keratin powder was prepared according to

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the method of Wawrzkiewicz et al. (1991).29 A reaction mixture (4.4 ml) containing 0.01 g of

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feather keratin powder and 200 µl of crude enzyme sample in glycine-NaOH buffer (pH 11.0)

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with 2 mM dithiothreitol (DTT) was incubated at 50 °C for 1 h, and the reaction was

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terminated by addition of 2 ml of 20 % trichloroacetic acid (TCA). The mixture was

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centrifuged at 12,000 ×g for 5 min, followed by the measurement of supernatant absorbance

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at 280 nm (A280) in a 1-cm cell. One unit of keratinolytic activity was defined as the amount

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of enzyme required to increase the A280 by 0.01 under the standard assay conditions (pH 11.0

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and 50 °C for 1 h).

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The protease activity was determined by using the Folin-phenol method of the People's

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Republic of China GB/T 23527-2009. Briefly, 0.5 ml of 1 % (w/v) casein solution was

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preheated at 40 °C for 10 min, followed by addition of 0.5 ml of 40 °C-preheated

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appropriately diluted enzyme solution. The reaction mixture was incubated at 40 °C for 10

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min, and 1 ml of 400 mM TCA was added to terminate the reaction. The reactions with

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enzyme addition after TCA were used as controls. After centrifugation at 13,000 ×g for 5 min,

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1 ml of the supernatant was added into a test tube containing 5 ml of 400 mM Na2CO3 and 1

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ml of Folin-phenol reagent, followed by incubation at 40 °C for 20 min. The absorbance was

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measured at 680 nm. One unit of protease activity was defined as the amount of enzyme that

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hydrolyzed casein to produce 1 µg of tyrosine per minute under the standard conditions (pH

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11.0 and 40 °C for 10 min).

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Characterization of Purified Recombinant KerK. Feather keratin powder was used as

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the substrate for biochemical characterization of recombinant KerK. The pH-activity profile

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of recombinant KerK was determined at 50 °C for 1 h in different buffers of pH 7.0−12.0. For

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pH stability assay, the enzyme was incubated at 37 °C in different buffers of pH 5.0−12.0 for 3

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h without substrate, and the residual enzyme activities were measured under standard

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conditions (pH 11.0 and 50 °C for 1 h). The buffers used were 50 mM of McIlvaine buffer

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(pH 5.0−6.0), 50 mM of Tris-HCl (pH 7.0−8.0), and 50 mM of glycine-NaOH (pH 9.0−12.0).

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The temperature-activity profile was determined by measuring the keratinase activity at

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different temperatures (30−70 °C) and optimal pH for 1 h. Thermal stability of KerK was

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determined by measuring the residual enzyme activities under standard conditions after

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incubation of the enzyme at 50 °C for 2, 5, 10, 20, 30 or 60 min without substrate.

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The effect of metal ions and chemical reagents on the activity of purified recombinant

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KerK was determined by adding 1 or 5 mM of various metal ions (Ca2+, Mg2+, Mn2+, Cr3+,

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Fe3+, Ni2+, Zn2+, and Co2+) and chemical reagents (5 mM of EDTA and PMSF and 1 %

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β-mercaptoethanol, SDS, Triton X−100, Tween-80, and Tween-20) to the assay system. The

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system without any addition of extra metal ions or chemical reagents mentioned above was

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treated as a control. Each reaction was run in triplicate.

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Chicken Feather Degradation by Purified Recombinant KerK in Vitro. Enzymatic 25

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degradation of feathers was conducted as described by Liang et al. (2010)

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modifications. Briefly, sterilized chicken feather (~5 cm) was incubated with 150 U of

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purified enzyme in 8 ml of 50 mM glycine-NaOH (pH 11.0) containing 2 mM DTT. The

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degradation degrees of feathers were observed at different time intervals, and enzymatic

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hydrolysis products were analyzed by HPLC-Chip/ESI-QTOF-MS in Institute of Apicultural

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Research, CAAS.

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Construction

of

B.

amyloliquefaciens

K11

Harboring

pUB110-kerK.

The

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electrocompetent cells of B. amyloliquefaciens K11 were prepared as described by Zhang et al.

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(2013).30 The plasmid pUB110-kerK was extracted from recombinant B. subtilis SCK6 using

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the TIANprep Midi Plasmid kit (Tiangen). Eighty microgram of competent cells were mixed

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with 5 µl of recombinant plasmid pUB110-kerK (~ 250 ng), kept on ice for 5 min, followed

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by electroporatation via a Bio-Rad Gene Pulser. The cells were then plated on LB plates with

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kanamycin (20 µg/ml). The positive transformants were verified by PCR and plated on 2 %

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(w/v) skim milk plates for preliminary screening of the protease activity.

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Degradation of Chicken Feathers and Production of Extracellular Protease. The

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engineered B. amyloliquefaciens K11 containing pUB110-kerK and parent strain B.

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amyloliquefaciens K11 were both grown in the feather medium with chicken feathers as the

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sole source of carbon and nitrogen. Their feather-degrading efficiency and halo sizes around

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the colonies were measured as described above for comparison. Meanwhile the keratinolytic

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and proteolytic activities of the culture supernatants were measured, respectively, as described

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above at different time intervals.

210 211

Nucleotide Sequence Accession Number. The nucleotide sequence for the B.

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amyloliquefaciens K11 keratinase gene (kerK) was deposited in the GenBank database under

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accession no. KR868996.

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RESULTS AND DISCUSSION

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The Feather Degrading Capacity of B. amyloliquefaciens K11. The keratinolytic

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potential of B. amyloliquefaciens K11 was assessed by growing the cells at 37 °C in the

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medium with chicken feathers as the only source of carbon and nitrogen. As results, visible

220

degradation of chicken feathers was observed after cultivation for 12 h, and complete feather

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degradation was achieved after 24 h (Fig. 1a). Usually, chicken feather degradation is a rather

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slow process, which takes most of strains at least 3−7 days for complete degradation. For

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example, B. subtilis SLC degraded feather completely at room temperature after 7 days,31 B.

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licheniformis PWD-1 degraded the feather keratin completely after 7 to 10 days at 50 °C,32

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and Thermoactinomyces sp. CDF took 72 h to degrade feather keratin completely.33 In

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comparison

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feather-degradation capacity and great potential in the disposal of waste feathers for industrial

228

purposes.

with

these

feather-degrading

strains,

strain

K11 showed

significant

229 230

Isolation of the Keratinase from B. amyloliquefaciens K11. The culture supernatants of

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B. amyloliquefaciens K11 were collected after 36-h incubation at 37 °C. SDS-PAGE analysis

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showed the presence of a major band of 27 kDa in the culture supernatants (Fig. 2a). The

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absence of other proteins might be ascribed to their very low expression levels or other

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proteins were degraded by the protease in the feather medium. Further LC-ESI-MS/MS

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analysis identified peptide fragments, VAVIDSGIDSSHPDLK, YPSVIAVGAVDSSNQR, and

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HPNWTNTQVR, which were 100 % identical to the putative protein sequence of B.

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amyloliquefaciens Y2.

238 239

Gene Cloning, Sequence analysis and Recombinant Plasmid Construction. The

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full-length keratinase gene (kerK) of B. amyloliquefaciens K11, 1149 bp in length, showed

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high identity (99 %) with that of B. amyloliquefaciens Y2. The Keratinase showed significant

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feather-degrading activity, and its encoding gene kerK was first reported in B.

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amyloliquefaciens. Sequence analysis showed that the kerK encoded a polypeptide of 382

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amino acids consisting of a signal peptide of 30 amino acids residues, a pro-sequence of 77

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residues and a mature protein of 275 residues, which shares a 63.7 % sequence identity with

246

the commercial keratinase from Bacillus licheniformis PWD-1.34 The kerK gene including the

247

native promoter and terminal sequence was then cloned into pUB110 vector by POE-PCR27

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and transforming into B. subtilis SCK6 to construct the recombinant plasmid pUB110-kerK.

249 250

Expression and Purification of kerK in B. subtilis SCK6. Due to the favorable

251

characteristics of secretion pathway, nonpathogenic and non-biased codon usage,22,24

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protease-deficient B. subtilis SCK6 was selected as the expression host for the kerK gene. The

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cells of recombinant B. subtilis SCK6 harboring pUB110-kerK produced halo zones when

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grown on LB plates containing 2 % skimmed milk, while those of B. subtilis SCK6 harboring

255

the empty vector (the control) didn’t (Fig. 3a). This indicated that kerK encodes an enzyme

256

which has protease activity as reported by Wang et al. (2015).33 When grew the recombinant

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cells at 37 °C in the feather medium, feathers were completely degraded at 24 h by B. subtilis

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SCK6 harboring pUB110-kerK while the control didn’t (Fig. 1b). These results indicated that

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kerK is the key gene for feather degradation. The successful expression of kerK in B. subtilis

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SCK6 endowed the strain SCK6 capacity to degrade feather efficiently. Recombinant KerK

261

was then purified to eletrophoretic homogeneity through ammonium sulfate precipitation. The

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purified protein migrated as a single band on SDS-PAGE with the molecular mass of about

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27.0 kDa (Fig. 2b).

264 265

Biochemical Characterization of Recombinant KerK. Most microbial keratinases are

266

neutral or alkaline proteases with the pH optima ranging from 7.5 to 9.0, except for a few

267

alkaliphilic enzymes from Kocuria rosea35 and Nocardiopsis sp. TOA-1.36 The purified

268

recombinant KerK is an extreme alkaline keratinase. Using feather keratin powder as the

269

substrate, purified recombinant KerK showed the maximum activity at pH 11.0 (Fig. 4a), and

270

retained stable over a wide pH range from 6.0 to 12.0 (Fig. 4b). These alkaline characteristics

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make KerK suitable for industrial applications in textile processing enzymatic depilation and

272

detergent37. The optimum temperature of recombinant KerK at pH 11.0 was 50 °C (Fig. 4c).

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And the enzyme lost 30 % enzyme activity after pre-treatment at 50 °C for 30 min (Fig. 4d),

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which was similar to that of commercial keratinase from B. licheniformis PWD-1.38

275

The effects of different metal ions and chemical reagents on the enzyme activity were

276

shown in Table 2. The activity of recombinant KerK was enhanced by Ca2+, Fe3+ and

277

β-mercaptoethanol, partially inhibited by Cr3+ and Mg2+, and strongly inhibited by Mn2+, Ni2+,

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Co2+, SDS, PMSF and EDTA. The stimulatory effect of Ca2+ might be ascribed to its

279

formation of salt or ion bridges, thus stabilizing the enzyme under its active conformation and

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protecting the enzyme against denaturation.39 The increased activity by β-mercaptoethanol

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may own to its reducing power for disruption of the disulfide bonds. However, the possible

282

mechanism of Fe3+ stimulation needs further study. The results suggested that the activity of

283

recombinant KerK will be enhanced by adding reducing agents and some metal ions in

284

industrial applications. Moreover, the strong inhibition of recombinant KerK by PMSF and

285

EDTA indicated that KerK belongs to serine-metallo-proteases10,33,40 and deduced KerK share

286

a 97 % sequence identity with B. amyloliquefaciens subtilisin (1ST2_A), showing that KerK

287

belongs to subtilisin family of serine proteases.

288 289

Degradation of Feathers by Purified KerK. Most purified keratinases cannot degrade

290

keratin by themselves.41 The process of feather degradation by keratinase is presumed to

291

consist of two basic steps, i.e. reduction of disulfide bonds42 and proteolysis that releases

292

short peptides and amino acids.5,43,11 Microbial feather degradation requires the cooperation of

293

reducing power provided by the cell themselves and extracellular secretion of proteolytic

294

enzymes.33,39,44,45 The feather-degrading capacity of purified recombinant KerK was assessed

295

in the presence of reducing agent DTT or not in this study. When DTT was included in the

296

enzyme reaction system, the chicken feather was completely degraded by purified

297

recombinant KerK within 0.5 h at 50 °C (Fig. 5). Without DTT, the degradation of chicken

298

feather was unobvious. This result indicated that the presence of reducing agent is necessary

299

for the efficient hydrolysis of feathers by purified recombinant KerK.

300 301

Analysis of Enzymatic Hydrolysis Products. The peptides with molecular masses

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below 3 kDa possess reduced allergenicity and are rich in many high-value bioactive

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components.46 HPLC-Chip/ESI-QTOF-MS analysis indicated that the peptides obtained from

304

feather hydrolysis have molecular masses of 1 to 3 kDa and have major cleavage sites at

305

serine

306

IQPSPVVVTLPGPILSS, and ILSEEGVPISSGGF as the main feather hydrolysis products

307

contained many essential amino acids, therefore would be applied for recycling utilization of

308

feathers, especially in feed industry as feed components.

and

hydrophobic

amino

acids.

Polypeptides

VVIQPSPVVVTLPGPILSS,

309 310

Comparison of the Feather-degrading Capacity and Keratinase Production by

311

Native and Recombinant B. amyloliquefaciens K11. Keratin-degrading microorganisms

312

have attracted much attention due to their extensive industrial applications.40 When incubated

313

in the feather medium, native B. amyloliquefaciens K11 completely degraded the feathers at

314

24 h (Fig. 1a). Under the same conditions, complete feather degradation was achieved by

315

recombinant B. subtilis SCK6 and recombinant B. amyloliquefaciens K11 at 24 h and 12 h

316

(Fig. 1b, 1c), respectively. Besides the highest efficiency of feather degradation, recombinant

317

B. amyloliquefaciens K11 was found to produce the biggest zone on skimmed milk plates (Fig.

318

3b), suggesting that it has the highest proteolytic activity.

319

During fermentation, the keratinolytic activity of recombinant B. amyloliquefaciens K11

320

harboring multiple copies of kerK increased gradually and reached the maximum (1500 U/ml)

321

at 60 h. This yield is significantly higher than that of the parent strain B. amyloliquefaciens

322

K11 (240 U/ml) (Fig. 3c and Table 3) and Thermoactinomyces sp. CDF (400 U/ml).33

323

Meanwhile it is noteworthy that the recombinant B. amyloliquefaciens K11 containing

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pUB110-kerK only took 12 h to degrade feathers completely and released many essential

325

amino acids. Therefore, the recombinant B. amyloliquefaciens K11 has great potential for

326

applications in waste feather disposals for poultry industry, additional, which has higher

327

keratinase expression level than others.

328

In conclusion, a B. amyloliquefaciens strain with significant feather-degrading capacity

329

was reported in this study. The functional gene for feather degradation, kerK, was then cloned

330

and expressed in B. subtilis SCK6. The purified recombinant KerK showed the maximum

331

activity at pH 11.0 and 50 °C, and degraded whole feathers within 0.5 h in the presence of

332

DTT. In comparison with native K11, recombinant B. amyloliquefaciens K11 containing the

333

plasmid pUB110-kerK had a 6-fold increase in keratinase production and degraded feathers

334

completely within 12 h. The efficient and rapid feather-degrading capacity of recombinant

335

strain K11 makes it valuable in waste feather disposal and recycling.

336 337

ACKNOWLEDGEMENTS

338 339

This research was supported by the “Twelfth Five-Year” National Science and

340

Technology Project in Rural Areas of China (2013BAD10B01-2) and the National High

341

Technology Research and Development Program of China (863 program, 2013AA102803).

342 343

Competing Interests:

344

The authors declare no competing financial interest.

345

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Figure legends

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Fig. 1 Visual observation of the chicken feather degradation by B. amyloliquefaciens K11 at

479

24 h (a), recombinant B. subtilis SCK6 at 24 h (b) and recombinant B. amyloliquefaciens K11

480

at 12 h (c).

481 482

Fig. 2 SDS-PAGE analysis of native and purified recombinant KerK. Lane M shows the

483

molecular mass standards, lane 1 shows the native KerK from the culture supernatants of B.

484

amyloliquefaciens K11, and lane 2 shows the purified recombinant KerK.

485 486

Fig. 3 Proteolytic and keratinolytic activities of parent B. amyloliquefaciens K11 and

487

recombinant B. subtilis and B. amyloliquefaciens K11 strains. a The halo zones of B. subtilis

488

SCK6 harboring empty vector (1) and pUB100-kerK (2) on LB agar containing 2 % skimmed

489

milk at 37 °C for 24 h. b The halo zones of parent strain B. amyloliquefaciens K11 (3),

490

recombinant B. subtilis SCK6 containing pUB100-kerK (4) and recombinant B.

491

amyloliquefaciens K11 (5). c Extracellular keratinolytic activities. Each value in the panel

492

represents the means ± SD (n = 3).

493 494

Fig. 4 Enzymatic properties of purified KerK. a Effect of pH on enzyme activities. The

495

enzyme assay was performed at 50 °C for 60 min. b pH stability. The enzyme was

496

pre-incubated without substrate at 37 °C for 3 h, and then subjected to residual activity assay

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under standard conditions (pH 11.0 and 50 °C for 60 min). c Effect of temperature on enzyme

498

activities determined at pH 11.0 for 60 min. d Thermostability. The residual enzyme activities

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were measured under standard conditions after pre-incubation of the enzyme without

500

substrate in glycine-NaOH (pH 11.0) for various periods. Each value in the panel represents

501

the means ± SD (n = 3).

502 503

Fig. 5 In vitro degradation of chicken feathers by purified KerK (150 U). Chicken feathers (5

504

cm) were incubated in 8 ml of 50 mM glycine-NaOH (pH 11.0) at 50 °C in the presence (+)

505

or absence (−) of 2 mM DTT. Images of feather degradation were captured at 0.5 h.

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Table 1. Oligonucleotide primers used in this study. Primers

Sequences (5´→3´)

kerKF

CTGACCGAGATTTTTTTGAGCAACTCGGGTTCCTATTAAACGAAAGA GAG

kerKR

CTTAGTGCTTTCATAGATTAAACTCTAAAAAAACCGGCGGGGCCA TGGCC

pUB110F

GGCCATGGCCCCGCCGGTTTTTTTAGAGTTTAATCTATGAAAGCACTA AG

pUB110R

CTCTCTTTCGTTTAATAGGAACCCGAGTTGCTCAAAAAAATCTCGGTC AG

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Table 2. The effects of different metal ions and chemical reagents on the enzyme activity of recombinant KerK. Chemicals Control

Concentrations

Relative activity (%) 100 ± 2



Ca2+

5 mM

129 ± 3

Mg2+

5 mM

75 ± 1

Mn2+

5 mM

11 ± 2

Fe3+

1 mM

211 ± 7

Cr3+

1 mM

92 ± 2

Zn2+

1 mM

36 ± 1

Co2+

1 mM

15 ± 1

Ni2+

1 mM

9±1

EDTA

5 mM

20 ± 2

PMSF

5 mM

2±1

β-Mercaptoethanol

1%

220 ± 7

SDS

1%

2±1

Triton X-100

1%

40 ± 1

Tween-80

1%

67 ± 3

Tween-20

1%

65 ± 1

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Table 3. Proteolytic and keratinolytic activities of keratinase KerK from native and recombinant B. amyloliquefaciens K11.

Substrate

Enzymatic activity (U/ml) Recombinant Native B. amyloliquefaciens K11 B. amyloliquefaciens K11

Feather keratin powder

238.4 ± 6.1

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1490 ± 24

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