Show bacteria build DNA with proteins
- Stanford researchers reported in Science that a bacterial anti-phage system called DRT3 can make new DNA with a protein-guided mechanism, not standard copying. - The team found Drt3b builds the poly(AC) half of a repeating GT/AC double helix without any nucleic-acid template in a 6:6:6 complex. - The work adds a new DNA-synthesis mechanism to biology’s toolkit and bacterial virus defense. (science.org)
Bacteria usually store viral “mugshots” in DNA using enzymes that copy an existing genetic template. A Science paper from Stanford reports one bacterial defense system can make sequence-specific DNA with a protein-guided step instead. (science.org) The system is called DRT3, short for a defense-associated reverse transcriptase found in bacteria that fight off phages, the viruses that infect bacteria. The Stanford team studied DRT3 from Escherichia coli and identified three parts: Drt3a, Drt3b, and a noncoding RNA. (science.org) (popularmechanics.com) In ordinary DNA synthesis, an enzyme reads a DNA or RNA template the way a scribe copies a line of text. Here, Drt3a still follows a template: it uses an ACACAC sequence embedded in the noncoding RNA to build a repeating poly(GT) strand. (science.org) The surprise comes on the matching strand. The paper says Drt3b makes the complementary poly(AC) strand “in the complete absence of a nucleic acid template,” using conserved amino-acid residues in its active site to keep the A-C-A-C pattern on track. (science.org) To see how that happens, the researchers used cryo-electron microscopy, a method that freezes molecules and images them at near-atomic scale. They resolved a D3-symmetric complex at 2.6 angstroms with six copies each of Drt3a, Drt3b, and the noncoding RNA. (science.org) That matters because polymerases are usually sorted into two camps: ones that copy a nucleic-acid template and ones that add loose, low-complexity sequences without one. The Science paper places DRT3 in a narrower category: sequence-specific DNA synthesis that is not directed by DNA or RNA base-pairing. (science.org) Stanford’s biochemistry department said the enzyme creates sequence-specific DNA repeats “using its own amino acids as the template.” Outside coverage described the result as a challenge to the standard rule that defined DNA sequence comes from a nucleic-acid template. (biochemistry.stanford.edu) (sciencealert.com) The work sits inside a fast-growing corner of microbiology: bacterial anti-phage systems. Researchers have been cataloging defense-associated reverse transcriptases for several years, but the Science paper says whether polymerases could generate sequence-defined DNA without a nucleic-acid template had remained unknown. (science.org) The finding does not mean cells have abandoned the usual rules of replication. It means one bacterial virus-defense pathway appears to use an extra trick, producing an alternating GT/AC double-stranded DNA product through a mixed mechanism: RNA-guided on one strand, protein-guided on the other. (science.org) For now, the clearest takeaway is narrower than the hype around it: Stanford’s team showed that DRT3 can build a very specific repeating DNA product in a way biologists had not documented before. The next step is figuring out exactly how bacteria use that product to stop phage infection. (science.org)
