The new understanding of retrons’ natural function could boost efforts to put them to work, according to an article published in ScienceMag.
Retrons and their efficiency
Defends bacteria
Last week in Cell, one team described how a specific retron defends bacteria, triggering newly infected cells to self-destruct so the virus can’t replicate and spread to others. The Cell paper “is the first to concretely determine a natural function for retrons,” says Anna Simon, a synthetic biologist at Strand Therapeutics who has studied the bacterial oddities. Another paper, which so far has appeared only as a preprint, reports a similar finding.
Efficient editing
The new understanding of retrons’ natural function could boost efforts to put them to work. Retrons are “quite efficient tools for accurate and efficient genome editing,” says Rotem Sorek, a microbial genomicist at the Weizmann Institute of Science and an author of the Cell study. But they don’t rival CRISPR yet, in part because the technology hasn’t been made to work in mammalian cells.
Background of the research
In the 1980s, researchers studying a soil bacterium were puzzled to find many copies of short sequences of single-stranded DNA littering the cells. Eventually they realized an enzyme called reverse transcriptase had made that DNA from the attached RNA, and that all three molecules—RNA, DNA, and enzyme—formed a complex.
Similar constructs, dubbed retrons for the reverse transcriptase, were found in many bacteria. “They really are a remarkable biological entity, yet nobody knew what they were for,” says Ilya Finkelstein, a biophysicist at the University of Texas, Austin.
Sorek came upon an early hint of their function when he and his colleagues searched through 38,000 bacterial genomes for genes used to fight off phages. One stretch of DNA stood out to Weizmann graduate student Adi Millman because it included a gene for a reverse transcriptase flanked by stretches of DNA that didn’t code for any known bacterial proteins. By chance, she came across a paper about retrons and realized that the mysterious sequences encoded one of their RNA components.
The research team went on to show that bacteria needed all three components—reverse transcriptase, the DNA-RNA hybrid, and the second protein—to defeat a variety of viruses.
Retron Ec48
About Ec48
For a retron called Ec48, Sorek and colleagues showed the associated protein delivers the coup de grâce by homing in on a bacterium’s outer membrane and altering its permeability. The researchers concluded that the retron somehow “guards” another molecular complex that is the bacterium’s first line of antiviral defense. Some phages deactivate the complex, which triggers the retron to unleash the membrane-destroying protein and kill the infected cell, Millman, Sorek, and their team reported on 6 November in Cell.
How it works
Led by Athanasios Typas, a microbiologist at the European Molecular Biology Laboratory (EMBL), Heidelberg, a second group realized that next to the genes coding for a retron in a Salmonella bacterium was a gene for a protein toxic to Salmonella. The team discovered the retron normally keeps the toxin under wraps, but activates it in the presence of phage proteins.
Earlier Researches
Other researchers had taken advantage of retrons’ then-mysterious features to devise new gene editors. CRISPR easily targets and binds to or cuts desired regions of the genome, but so far it isn’t very adept at introducing new code in the target DNA. Retrons, combined with elements of CRISPR, can manufacture lots of copies of a desired sequence, which can be spliced efficiently into the host genome.
In 2018, researchers in Hunter Fraser’s Stanford University lab made retrons whose RNA matched yeast genes, but with one base mutated. They combined them with CRISPR’s “guide RNA,” which homes on the targeted DNA, and the CAS9 enzyme that acts as CRISPR’s molecular scissors. Once CAS9 cut the DNA, the cell’s DNA repair mechanisms replaced the yeast gene with the DNA generated by the retron’s reverse transcriptase.
“It could be that retrons will be as revolutionary as CRISPR has been,” Simon says. “But until we understand more about the natural biology and synthetic behavior of retrons, it is difficult to say.”
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Source : SceinceMag
