Stanford researchers have discovered a bacterial immune enzyme that writes genetic code without using any DNA or RNA as a guide, upending a fundamental assumption about how living cells copy nucleic acids.
For decades, molecular biology has been anchored by a central rule: sequence-specific DNA or RNA synthesis requires a template. Polymerases depend on Watson–Crick base pairing, reading an existing strand to guide the incorporation of complementary nucleotides. In the absence of such a template, synthesis was thought to be inherently random or restricted to short, predefined motifs. This new study fundamentally challenges that paradigm by demonstrating that biological systems can encode and transfer genetic information without a nucleic acid template.
An Immune System With an Unusual Weapon
The enzyme complex, DRT3, belongs to a growing class of bacterial defense systems that protect against bacteriophage infection. These defense-associated reverse transcriptases (DRTs) have emerged as a remarkably diverse group, employing unconventional biochemical strategies to counter viral threats. While some generate random DNA polymers and others construct repetitive sequences, DRT3 stands out for its unique architecture and function.
Unlike typical systems, DRT3 is composed of two evolutionarily distinct reverse transcriptase proteins, Drt3a and Drt3b, alongside a small noncoding RNA. Although previous studies established that all three components are essential for antiviral defense, the underlying mechanism remained unclear until now.
A Machine Built for Two
Structural and biochemical analyses revealed that DRT3 assembles into a highly organized hexameric complex, containing six copies each of Drt3a, Drt3b, and the RNA component. This molecular machine exhibits a clear functional division between its two catalytic subunits.
Drt3a operates in a manner reminiscent of telomerase. It reads a short RNA motif (ACACAC) and uses it as a template to synthesize long single-stranded DNA composed of repeating GT units. This templated activity aligns with established principles of nucleic acid synthesis and reflects a conserved strategy seen in other biological systems.
In contrast, Drt3b performs a function that defies conventional understanding.
Writing DNA Without a Blueprint
Drt3b synthesizes a complementary poly(AC) DNA strand without relying on any nucleic acid template. Structural data confirm that no RNA or DNA strand is present near its active site, and the enzyme’s template-binding channel is physically blocked by its own structural elements.
Instead of base pairing, Drt3b uses its own amino acid residues to dictate nucleotide incorporation. The growing DNA strand adopts a distinct conformation within the active site, where protein side chains enforce sequence specificity through hydrogen bonding, steric constraints, and stacking interactions.
Two residues are central to this process: Glu26 selectively stabilizes adenine incorporation, while Arg253 enforces cytosine addition at alternating positions. Mutations in these residues disrupt sequence fidelity and abolish the bacterium’s antiviral defense, demonstrating that protein-templated synthesis is both precise and biologically essential.
How Unprecedented Is This?
Although limited examples of protein-guided nucleotide addition exist, they are far less complex. Enzymes such as the CCA-adding enzyme or DNA polymerase Rev1 incorporate only a few nucleotides using protein-based specificity. Even systems that generate repetitive RNA sequences lack the structural resolution and mechanistic clarity observed here.
Drt3b, by contrast, produces kilobase-length DNA with strict sequence patterning, entirely directed by protein structure. This represents a previously unrecognized mode of biological information transfer, extending far beyond known exceptions.
The Trigger: A Phage Protein
The study also identifies how DRT3 is activated. Through adaptive evolution experiments, researchers traced phage resistance to mutations in a single viral protein, ST61. When this protein is detected, DRT3 initiates a toxic response that kills the infected bacterial cell, preventing viral replication.
This mechanism places DRT3 within a class of “guard” immune systems, which respond to specific viral signatures. However, the precise role of the synthesized poly(GT/AC) DNA in mediating toxicity remains unresolved. It may act by sequestering viral proteins or forming higher-order DNA structures that disrupt replication.
What It Could Mean Beyond Bacteria
The implications of this discovery extend into multiple fields. In biotechnology, it introduces the possibility of engineering polymerases whose sequence specificity is encoded entirely within protein structure, eliminating the need for templates. Such systems could enable controlled synthesis of defined DNA sequences in new ways.
In synthetic biology, the repetitive DNA products generated by DRT3 possess unique structural properties, including the potential to form noncanonical conformations such as Z-DNA. These features could be leveraged for constructing DNA-based materials or molecular scaffolds.
From an evolutionary perspective, the findings blur the boundary between templated and non-templated synthesis, suggesting that early biochemical systems may have exploited similar mechanisms.
A Field Accelerating
The rapid expansion of the DRT family underscores how much remains to be discovered. Each newly characterized system reveals unexpected biochemical capabilities, and the majority of identified DRTs remain unexplored.
The discovery of protein-templated DNA synthesis does more than redefine a specific bacterial defense mechanism, it reshapes a foundational concept in molecular biology. It demonstrates that life can encode genetic information through protein structure alone, expanding the known repertoire of biological information processing.
Reference
Deng, P et al. Protein-templated synthesis of dinucleotide repeat DNA by an antiphage reverse transcriptase.Scienc, 6 Apr 2026, First Release, DOI: 10.1126/science.aed1656
Image credit: Portions of the figure in this article were generated using ChatGPT (OpenAI) or Google Gemini.
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