Bacterial enzyme reported to synthesize DNA

Stanford University researchers reported in Science on April 17 that a bacterial defense system called DRT3 can build double-stranded DNA using two different mechanisms, one of them unlike standard template-based DNA synthesis. The paper, by Pujuan Deng, Hyunbin Lee, Carlo Armijo and colleagues, described DRT3 as a three-part system made of two reverse transcriptases — Drt3a and Drt3b — plus a noncoding RNA. (science.org) The authors said Drt3a makes one strand in the familiar way, by reading an RNA sequence embedded in that noncoding RNA. The surprise, they reported, is that Drt3b makes the matching strand without a DNA or RNA template, using features of the protein itself to control which nucleotides are added. ### Which enzyme is doing the unusual step? Drt3b is the subunit the paper identifies as carrying out the nonstandard reaction. The Science article said Drt3b synthesizes a complementary poly(AC) DNA strand “in the complete absence of a nucleic acid template,” while Drt3a produces the poly(GT) partner strand from a conserved ACACAC sequence in the system’s noncoding RNA. The product the team described is alternating poly(GT/AC) double-stranded DNA. That matters because the authors framed the finding not as generic template-free tailing, which enzymes can already do in limited ways, but as sequence-specific synthesis with a defined alternating pattern. (science.org) ### How did the researchers say the system works? Cryo-electron microscopy at 2.6 angstrom resolution showed what the paper described as a D3-symmetric 6:6:6 complex of Drt3a, Drt3b and the noncoding RNA. The authors said those structures, together with biochemical experiments, support a model in which Drt3a reads the RNA template and Drt3b uses conserved active-site residues to enforce the alternating A and C pattern on the opposite strand. (science.org) Science summarized the finding as a “protein-templated mechanism for sequence-specific DNA synthesis.” That wording is narrower than saying all DNA can now be made without templates: the reported mechanism applies to this bacterial antiphage system and to the specific repetitive product described in the paper. (science.org) ### Where does this fit in what biologists already knew? The Science paper said nucleic-acid polymerases have generally been grouped into template-directed enzymes, which copy existing DNA or RNA, and template-independent enzymes, which usually make simpler products such as homopolymers or short motifs. (science.org) The authors wrote that distinct modes of nucleotide polymerization may still exist among poorly characterized proteins, particularly in bacterial defense systems. Phys.org, citing the study, reported that DRT3 systems were found in at least 20 bacterial species and that the researchers introduced DRT3 into E. coli to test how it behaved during phage attack. (science.org) In those experiments, the outlet said, the system generated long repetitive DNA with a specific alternating pattern and was activated by detection of a phage protein called ST61. ### Is this the same thing as rewriting the “central dogma”? No. The paper concerns how an enzyme synthesizes DNA, not the broader flow of information from DNA to RNA to protein. What the authors reported is a new route for sequence-specific DNA synthesis inside a bacterial antiviral pathway, with one strand guided by RNA and the other by protein features in Drt3b. (science.org) The authors themselves used more limited language. They wrote that the findings “expand the functional landscape of nucleic acid polymerases,” rather than claiming that standard DNA replication rules no longer apply across biology. (phys.org) ### Why are origin-of-life and synthetic-biology researchers paying attention? Science said the work identifies a protein-templated route to sequence-specific DNA synthesis, a result that could inform experiments on how early information-bearing molecules formed and how enzymes might be adapted for DNA writing. That is a research direction, not a product claim: the paper reports a bacterial mechanism and structural explanation, not a ready-made tool for manufacturing arbitrary DNA sequences. (science.org) The next step is likely to come from follow-up work on DRT3 and related defense-associated reverse transcriptases. (science.org) The peer-reviewed record so far is the April 17 Science paper by Deng and colleagues, which contains the structural data, sequencing results and methods behind the claim.