SYNTHETIC BIOLOGY
Biologists evolve a proofreading reverse transcriptase The enzyme copies RNA into DNA without making mistakes One of the hallmarks of many DNA polymerases is that they proofread as they copy DNA, ensuring high-fidelity duplicates of an organism’s genome. This error-checking ability is missing from enzymes called reverse transcriptases, which copy RNA sequences to DNA. At least that was the case until a team led by Jared W. Ellefson and Andrew D. Ellington at the University of Texas, Austin, used directed evolution to convert a DNA polymerase to a proofreading reverse transcriptase (Science 2016, DOI: 10.1126/ science.aaf5409). The new enzyme might find use in labs wishing to do accurate sequencing of RNAs in cells, Ellefson says. Normally, this process involves converting RNA to DNA, which gets amplified using polymerase chain reaction techniques. If researchers are analyzing RNA from a single cell, any mistakes in the conversion from RNA to DNA also get amplified. “This enzyme could be a game changer for single-cell analyses,” comments Vitor Pinheiro, who engineers DNA polymerases at University College London.
Reverse transcriptases catalyze nearly the same reaction as do DNA polymerases. Both connect up DNA bases following a nucleic acid template—it’s just the template that differs. So the researchers had to evolve a DNA polymerase that reads an RNA template instead of a DNA one. To do so, the team introduced a library of randomly mutated DNA polymerases into Escherichia coli cells. Then they inserted a few RNA bases into strings of DNA that are necessary for the enzymes’ function, forcing the DNA polymerases to develop reverse transcriptase skills. The researchers repeated the process, increasing the number of RNA bases, until the bacteria’s DNA polymerases could fully read RNAs while maintaining their proofreading skills. The conversion required about a dozen amino acid mutations in the DNA polymerase. The work also resolved a long-standing conundrum in evolutionary biochemistry, Ellefson says. Namely, why are reverse transcriptase enzymes inherently error-prone, while many DNA polymerases have exquisite accuracy? The relatively minor changes
This enzyme was evolved from a DNA polymerase into a reverse transcriptase. Residues essential (red) and important (pink) for the new functionality are highlighted. The template RNA strand is yellow, and the DNA strand is blue. needed to evolve an error-prone reverse transcriptase suggest that sloppy copying may confer evolutionary advantage to organisms using the enzymes. For example, HIV uses reverse transcriptases to copy its RNA genome into DNA to then insert into a host’s genome. Copy errors likely enable HIV’s rapid evolution, allowing the pathogen to stay one step ahead of human immune systems, Pinheiro adds. The work also suggests that DNA polymerases could be evolved to accommodate other nucleic acids besides RNA—perhaps synthetic ones. However, Pinheiro cautions that synthetic biologists may not want to give engineered organisms the ability to convert synthetic nucleic acids to DNA, thereby keeping them separate from the natural world.—SARAH EVERTS
MOLECULAR ELECTRONICS
CREDIT: AAAS/ SCIENCE
Improving flexible memory devices organically Flexible organic memory devices are now storing more information using light and divulging those data with electronic current. These aren’t the first organic devices to use different physics for writing and reading data, but earlier devices were slower, stored fewer data, and degraded more quickly, say Paolo Samorì and Emanuele Orgiu of the University of Strasbourg. Chemistry has now allowed Samorì, Orgiu, Stefan Hecht of Humboldt University of Berlin, and their colleagues to overcome these limitations to build
devices that could enable flexible, data-storing optoelectronic sensors and wearable electronics. The team developed a blend of light-sensitive S S diarylethene molecules Open and poly(3-hexylthiophene), a semiconducting polymer. Visible UV When excited by ultraviolet light, diarylethene molecules change from an open form to a closed one. This chemistry allows the team to quickly “write” information in the molecular S
S
Closed
The opening and closing of a diarylethene molecule.
blend with nanosecond-long laser pulses. The diarylethene molecules can be reopened with visible light to erase data. The ratio of opened to closed molecules influences how the poly(3-hexylthiophene) shuttles electronic charge, measured as a current. Each light pulse changes the current by a distinct interval, creating devices with at least 256 discrete current levels or memory states, many more than previously achieved with similar devices (Nat. Nanotechnol. 2016, DOI: 10.1038/ nnano.2016.87). Furthermore, the new devices can reliably store data for hundreds of days, even when disconnected from a power source, the team says.—MATT DAVENPORT DATE | CEN.ACS.ORG | C&EN
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