UC San Diego Study Shows RNA Polymerase Reads an Eight-Letter Genetic Alphabet

Every organism on Earth encodes its biology with the same four-letter genetic alphabet. Two papers out of UC San Diego now show that nature's molecular machinery isn't necessarily locked into that constraint.

The primary study, published September 2 in Nature Communications, reports that RNA polymerase, the enzyme that reads DNA and produces the RNA transcripts that drive gene expression, can accurately transcribe a doubled, eight-letter genetic alphabet known as Hachimoji. A companion paper, published August 12 in the Proceedings of the National Academy of Sciences, extended the finding to a separate synthetic base pair that lacks hydrogen bonds, which are generally considered essential to how DNA base pairs hold together and are recognized.

Both studies were led by Dong Wang, a professor at the UC San Diego Skaggs School of Pharmacy and Pharmaceutical Sciences.

The central question the team set out to answer was whether existing cellular machinery could handle synthetic genetic letters at all, or whether it would stall, misread, or drop fidelity. According to a UC San Diego news release reviewed by ETL Newswire, the answer leaned toward "handle it."

To get there, the researchers combined biochemical assays with high-resolution cryo-electron microscopy. The imaging work captured structural snapshots of E. coli RNA polymerase mid-transcription as it encountered synthetic base pairs. What the snapshots showed was not a dramatically altered recognition mechanism. Instead, as reported in Nature Communications, the enzyme appears to identify synthetic DNA letters through many of the same biochemical and structural signals it uses for natural ones.

That mechanistic similarity is the most scientifically interesting result here, and it also carries the most interpretive weight. A finding like this can look cleaner than it is because the experimental system is E. coli RNA polymerase in a controlled biochemical context. The question of whether expanded-alphabet transcription holds up in a living cell, across the full complexity of chromatin structure, competing factors, and proofreading machinery, is not answered by this work. The authors used a bacterial model, and the leap to eukaryotic gene expression would require its own set of experiments.

The PNAS result is, if anything, more mechanistically provocative. The finding that RNA polymerase can recognize a synthetic base pair without relying on hydrogen bonding challenges a fairly foundational assumption about transcription fidelity. Hydrogen bonding is considered one of the primary signals the enzyme uses to check that it has the right incoming nucleotide before catalyzing the addition. That the enzyme can manage without it, at least for these synthetic substrates, is the kind of result that requires replication and scrutiny before the field will absorb it comfortably.

Practical applications mentioned by the UC San Diego team include engineering biological systems that perform functions or produce compounds not available in natural organisms, as well as potential uses in biotechnology and medicine. Those are reasonable downstream ambitions, but the distance between demonstrating transcription fidelity in vitro and deploying an eight-letter genetic system in a therapeutic context is measured in years and unsolved problems.

What the papers actually establish, taken together, is a structural and biochemical foundation: the enzyme can do this, and the imaging now shows at near-atomic resolution why. That's a real and specific contribution. Calling it a rewrite of life's operating code would be reading well past the methods section.

Sources cited:
- UC San Diego News Release (today.ucsd.edu) (https://today.ucsd.edu/story/breakthrough-helps-expand-genetic-alphabet)
- Nature Communications (Hachimoji study, Sept. 2, 2026) (https://www.nature.com/articles/s41467-026-76668-0)
- Proceedings of the National Academy of Sciences (PNAS, Aug. 12, 2026) (https://doi.org/10.1073/pnas.2607774123)
- ScienceDaily coverage (https://www.sciencedaily.com/releases/2026/09/260904000310.htm)
- Sense About Science coverage (https://www.senseaboutscience.org.uk/rna-polymerase-shown-to-read-eight-letter-genetic-alphabet-in-uc-san-diego-studies/)