Scientists find bacteria DNA trick
- Stanford researchers and collaborators reported in Science that a bacterial anti-phage system called DRT3 builds double-stranded repeat DNA in an unrecognized way. - One enzyme copies a short RNA cue into poly(GT), while its partner makes matching poly(AC) without any nucleic-acid template in the complex. - The work extends a fast-growing map of bacterial virus defenses beyond CRISPR and restriction enzymes. (science.org)
Bacteria spend their lives under attack from bacteriophages, viruses that infect microbes, and they carry immune systems that try to stop those invasions. In a paper published in *Science* last week, researchers reported one of those systems makes DNA in a way not previously seen. (science.org) The system is called DRT3, short for defense-associated reverse transcriptase 3. The team found it in *Escherichia coli* and showed it helps the bacterium resist phage infection by producing a repeating double-stranded DNA molecule. (science.org) Reverse transcriptases usually copy RNA into DNA by reading a nucleic-acid template, like a scribe copying from a page. In DRT3, one enzyme, Drt3a, follows that familiar rule and uses a conserved ACACAC sequence in a noncoding RNA to build a poly(GT) strand. (science.org) The surprise came from the partner enzyme, Drt3b. The paper says Drt3b builds the matching poly(AC) strand in the complete absence of a nucleic-acid template, using conserved amino-acid residues in the protein itself to enforce the alternating pattern. (science.org) (stanford.edu) That means the bacterium is not just copying genetic material in the usual way. It is using a protein-guided mechanism to make sequence-specific DNA repeats, a result the authors said expands what nucleic-acid polymerases are known to do. (science.org) The researchers used cryo-electron microscopy to map the DRT3 machinery at 2.6 angstrom resolution. They described a 6:6:6 complex made of Drt3a, Drt3b, and a noncoding RNA, arranged with D3 symmetry. (science.org) This study lands in the middle of a broader rewrite of how bacterial immunity works. Over the past several years, researchers have shown that bacteria use many anti-phage systems besides CRISPR, including reverse-transcriptase systems that make unusual DNA products during infection. (science.org 1) (science.org 2) One recent example, DRT9, was shown to make long poly(A)-rich single-stranded DNA, up to about 5,000 nucleotides, when phage infection raises cellular deoxyadenosine triphosphate levels. DRT3 adds a different strategy: building alternating poly(GT/AC) double-stranded DNA with one strand made without a nucleic-acid template. (science.org 1) (science.org 2) Stanford’s biochemistry department said the paper shows a bacterial reverse transcriptase creating sequence-specific DNA repeats using its own amino acids as the template. The next step is not a new headline claim but the harder question the paper leaves open: exactly how that DNA product blocks phage infection inside cells. (stanford.edu) (science.org)
