NATURE BIOTECHNOLOGY
ResearchMWatch New strategy for biotransforming TNT Reducing one of the nitro groups on 2,4,6-trinitrotoluene (TNT) to an amine could render the explosive much easier to bioremediate with a bacterial oxygenase, report Jim Spain and researchers at the U.S. Air Force Research Laboratory and the University of Connecticut. They describe a scheme in which an amino form of TNT can continue decomposition through an oxidation pathway. Currently, incineration is the best method to remove toxic and explosive TNT from contaminated sites, but the approach is expensive and causes air pollution. Bioremediation would be an attractive alternative, but it appears that bacterial oxygenase enzymes cannot attack TNT directly. Spain and his colleagues speculated that the problem is that the nitro substituents in TNT withdraw too much electron density from the ring, inhibiting efficient enzymatic attack on the molecule. Using a bacterial reductase, the researchers generated aminodinitrotoluene (ADNT) isomers, a typical breakdown product which is known to have more electron density than TNT. According to a rate study, ADNT is a much more likely substrate for nitroarene dioxygenase attack. These findings expand the nitroarene dioxygenases’ capability beyond known substrates like mono- and di-nitrotoluenes and exhibit potential for better TNT remediation. (Appl. Environ. Microbiol. 2001, 67, 5460–5466)
GM plants transform TNT Researchers have shown that a genetically modified (GM) tobacco plant successfully transforms levels of 2,4,6-trinitrotoluene (TNT) found in contaminated soil. Neil Bruce and colleagues at the University of Cambridge and the Defence Science and Technology Laboratory at Fort Halstead in the United Kingdom re-
(Left)W ildtype and (right)GM tobacco seedlingsgerm inated in a TNT solution.
port that transgenic tobacco seedlings growing in a 0.25 millimolar solution of TNT gained 18% in wet biomass weight over seven days, while wildtype plants lost 34% weight. Moreover, no TNT was extracted from the transgenic seedlings, indicating that the explosive was completely transformed or stored as an inextractable form in the plant. Unexploded TNT in soil poses a safety hazard and a serious toxic environmental threat to groundwater and soil health. Incineration, the current best remediation procedure, can lead to other environmental problems, such as air pollution from incomplete combustion. Thus, a phytoremediation approach that can fully transform TNT significantly reduces the safety and environmental problems in cleaning up this contaminant. To modify tobacco to handle TNT, the scientists introduced a gene encoding for a nitroreductase enzyme that converts TNT to aminonitrotoluene compounds, which can then undergo further reactions. The nitroreductase gene, known as nsfI, was taken from the bacteria Enterobacter cloacae. (Nat. Biotechnol. 2001, 19, 1168–1172)
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Selenium speciation important for phytoremediation Efforts to develop phytoremediation strategies for naturally occurring selenium, which is toxic or teratogenic, have been hindered by the lack of sensitive methods that differentiate between various forms of the element. Joseph Caruso and colleagues at the University of Cincinnati report a new method that successfully identifies various inorganic selenium species, as well as biotransformation products like selenomethionine, a form of amino acid. Researchers have found that some wild-type plants, such as Brassica juncea, or Indian mustard, can accumulate selenium at levels as high as thousands of parts per million. However, in order to best use these plants for remediating contaminated soils, researchers first need to identify and quantify specific forms of selenium used by the plants. Previous analyses report only total selenium content. Caruso’s group used reversedphase liquid chromatography and inductively coupled plasma mass spectrometry to detect and identify parts-per-billion concentrations of selenium species transformed by the plant. Further studies using electrospray mass spectrometry determined a previously unidentified selenium– sulfur bridged amino acid with a proposed structure similar to cysteine. (Anal. Chem. 2001, 74, 107–113)
Forest fragments magnify environmental stresses The edges of deciduous forest fragments are hot zones of atmospheric deposition of pollutants and nutrients, according to new data by Kathleen Weathers and colleagues at the Institute of Ecosystem Studies in Millbrook, N.Y. The findings add to evidence that forests chopped into fragments by logging and road building amplify the effects of environ-
mental disturbances such as air pollution. Atmospheric deposition is often the largest source of nutrients and pollution delivered to forest ecosystems. Measurement of deposition below the canopy reveals that the edges of low-elevation deciduous forests in New York’s Hudson Valley boost sulfur inputs by 17%, belowcanopy dissolved inorganic nitrogen fluxes by 43%, and below-canopy calcium fluxes by 57% over levels in interior zones, Weathers reports. When the researchers removed all of the vegetation lower than one-half the canopy height at the forest edge, making the structure of the edge and interior comparable, they found that below-canopy deposition of nitrogen and sulfur was slightly greater in the forest interior.
deposition of pollutants below the canopy. The broad distribution of low-elevation forests and the growing trend of forest fragmentation suggest this phenomenon could have a significant impact on ecosystem processes (Conserv. Biol. 2001, 15, 1506–1514).
DIGITAL VISION
Global warming triggers gene change
Logging and road building in deciduous forestsenhancesatm ospheric deposition ofpollutants.
The existence of the abrupt forest edge, as well as its dense structure of lateral branches and shrubs, traps horizontally driven air pollutants, Weathers says. This edge enhancement of critical pollutants and nutrients—sulfur contributes to acid rain and forest growth is limited by nitrogen—could influence soil microbial activity and seedling survival, Weathers says. Although previous studies have demonstrated similar edge effects in high-elevation coniferous forests, this is the first study in the United States to show that the edges of low-elevation deciduous forests magnify the
Longer summers associated with global warming are causing some mosquitoes to evolve so that they enter their winter dormancy later, according to researchers at the University of Oregon. These findings provide the first example of a genetic shift for a seasonally related trait, they claim. William Bradshaw and Christina Holzapfel studied the pitcher-plant mosquito, Wyeomyia smithii, under laboratory conditions. These insects survive the winter by entering their pupae and lying dormant until spring. Day length (photoperiod), an indicator of seasonal change, drives mosquito behavior, the authors say. That is, shortening days trigger the onset of dormancy. The team found that the mosquitoes now enter their dormant phase 8 to 10 days later than they did in the 1970s. The shift was detectable over a time interval as short as five years. Bradshaw and Holzapfel believe that those insects that have adapted to longer summers by delaying dormancy until the days are shorter, can stay active for longer. “This response to day length is a highly heritable trait that is subject to natural selection. Populations are responding genetically, that is, evolving, as a consequence of selection for shorter critical photoperiods due, presumably, to longer growing seasons generated by climate warming,” says Bradshaw. According to the authors, variation in populations has two sources: genetic and environmental. They say that by controlling conditions in the lab, they factored out all possible environmental sources of variation, revealing the genetic differences. The insects that enter dormancy later have an advantage over the others and can pass this on to their offspring. They are able to continue to reproduce longer, leaving more offspring, and can accumulate more nutritional reserves. (Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 14,509–14,511)
Arsenic: The missing link? Researchers have discovered a mechanism that may explain the link between long-term exposure to arsenic in drinking water and cancer of the bladder, lungs, skin, kidney, nasal passages, liver, and prostate. Paradoxically, the mechanism that empowers arsenic as a tumor promoter also seems to be closely linked to arsenic’s effectiveness in treating some forms of leukemia. The findings could have implications for the U.S. EPA’s new 10 partsper-billion (ppb) rule for arsenic in drinking water. Although the lowest concentration of arsenic trioxide used in this latest experiment by WenChien Chou of Johns Hopkins University and colleagues was 20 ppb, the researchers say this same mechanism could play a role in chronic exposure to arsenic at very low levels. Studying arsenic trioxide’s exposure on various cell lines, including leukemia, cervical cancer, and breast cancer, Chou and colleagues observed a marked decrease in telomerase activity. Telomerase is an enzyme that maintains the length of chromosomal ends, or telomeres, which otherwise would become progressively shorter after each cell division. For most advanced tumors, telomere maintenance is essential for continued proliferation. In healthy, noncancerous cells, however, the loss of telomeres could lead to genomic instabilities and the formation of cancerous cells. The researchers found that arsenic inhibits the transcription of the human gene, hTERT, which in turn inhibits telomerase expression. The inhibition was dose-related and occurred with arsenic concentrations at or lower than those measured in plasma taken from human subjects exposed to the metal in pharmacokinetic studies. The authors believe that the inhibition of hTERT is the result of decreased activity of c-Myc and Sp1 transcription factors—proteins that promote the conversion of DNA sequences into corresponding RNA—in the exposed cultures. However, this explanation leaves open the questions of why other processes in which Sp1 plays a role are less affected by arsenic and whether the inhibitory effect of arsenic on other genes may also contribute to the decrease in hTERT expression and telomere maintenance. (J. Clin. Invest. 2001, 108, 1541–1547).
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