Researchers report protein builds DNA without template

Stanford University researchers reported on April 16 in *Science* that a bacterial anti-phage system can build one strand of DNA without using DNA or RNA as a template. The study describes a defense system called DRT3, found in bacteria, that makes long double-stranded DNA with alternating GT and AC repeats. One enzyme in the system follows a conventional RNA template, but a second enzyme, Drt3b, builds the matching strand without any nucleic-acid guide, the paper said. Alex Gao, the Stanford biochemist who led the work, told *Science* that the finding was “a fundamentally new way that life produces DNA.” ### Which protein is doing something biologists did not expect? DRT3 contains two reverse transcriptases, Drt3a and Drt3b, plus a noncoding RNA, the paper said. Drt3a reads a conserved ACACAC sequence in the RNA and uses it to produce a poly(GT) DNA strand, which fits the standard idea of template-directed synthesis. Drt3b then produces the complementary poly(AC) strand, but the authors said it does so “in the complete absence of a nucleic acid template.” (science.org) Cryo-electron microscopy structures at 2.6 angstrom resolution showed a D3-symmetric 6:6:6 complex of Drt3a, Drt3b and the noncoding RNA, according to the paper. The authors said conserved residues in Drt3b’s active site enforce the alternating base pattern, letting the protein’s own structure act as the guide for sequence-specific synthesis. ### How is this different from ordinary DNA copying? (science.org) DNA polymerases usually copy an existing DNA or RNA sequence, and template-independent enzymes generally make simpler products such as homopolymers or short motifs, the *Science* paper said. The authors wrote that Drt3b expands that landscape by generating a defined alternating sequence without reading a nucleic-acid template. (science.org) Alex Gao told *Science* that “the protein itself serves as a template for the DNA sequence.” In the account published by the Centre for Genomic Regulation’s communication office, Philip Kranzusch of Harvard Medical School called the work “groundbreaking,” while Adi Millman of MIT said using a protein as a template was “a significant conceptual shift,” comments that were presented as outside reaction to the paper. (science.org) ### Where did the researchers find this system? The supplementary methods said the team searched the NCBI nonredundant protein database in February 2025 and, after manual curation, identified 2,269 intact DRT3 systems. To reduce redundancy, the researchers clustered Drt3b sequences into 1,232 groups, the supplement said. (cobcv.com) Phys.org, citing the study, reported that DRT3 was found across at least 20 bacterial species and that Drt3a and Drt3b usually appear together. The main paper and supplement identify the lead institution as Stanford University, with authors from the university’s departments of biochemistry, microbiology and immunology, biology, and Sarafan ChEM-H. (science.org) ### What does DRT3 do for bacteria? The paper describes DRT3 as a defense-associated reverse transcriptase system that helps bacteria resist phage infection. Phys.org reported that the researchers introduced DRT3 into *E. coli* and exposed the bacteria to phages, and that the system was activated by a phage protein called ST61. The mechanism of protection is still unresolved. (phys.org) Gao told *Science* that one possibility is that the unusual DNA products act as molecular sponges that bind phage components or help other immune factors recognize infection, but he presented that as a hypothesis rather than a demonstrated mechanism. (science.org) ### Is this the only such system researchers have reported? A separate preprint posted on April 18 by researchers at Southern University of Science and Technology in Shenzhen described another bacterial defense enzyme, DRT7, that also synthesizes protein-templated DNA. That study has not been peer reviewed, but it reported that a reverse transcriptase domain initiates sequence-specific poly(T) synthesis without a complementary nucleic-acid template. (cobcv.com) The Stanford study is the peer-reviewed paper tied to the May social-media summary, and it appeared in *Science* as “Protein-templated synthesis of dinucleotide repeat DNA by an antiphage reverse transcriptase,” DOI 10.1126/science.aed1656. The supplementary file lists additional datasets and methods, and the corresponding author is Alex Gao at Stanford. (science.org) (biorxiv.org)