UC San Diego Shows E. coli RNA Polymerase Reads Hachimoji 8-Letter DNA via Cryo-EM

The Nature Communications study combined biochemical assays with high-resolution cryo-electron microscopy to capture RNA polymerase from E. coli as it bound and incorporated Hachimoji bases. A parallel PNAS paper examined a hydrophobic unnatural pair lacking hydrogen bonds yet still triggered catalysis through trigger-loop closure. Sample sizes in the structural datasets exceeded 100,000 particles per complex, with resolutions reaching 2.8 Å. This design isolates polymerase fidelity mechanisms without requiring full cellular replication.

The work extends earlier synthetic DNA applications that detected liver cancer cells by showing cellular machinery can process non-natural letters without new enzymes. It challenges assumptions that hydrogen bonding is essential for transcription fidelity and links directly to efforts expanding the genetic code for novel amino acids or diagnostics. The finding matters because it lowers the barrier to engineering organisms that produce compounds absent from natural biology.

Key limitation is the in vitro focus on purified polymerase; no data yet confirm stable propagation or translation in living cells. Full validation would require constructing an 8-letter plasmid that replicates and expresses functional proteins inside E. coli or a minimal synthetic cell. Such experiments would test whether downstream ribosomes and tRNA synthetases tolerate the expanded alphabet at scale.

Next steps include testing eukaryotic polymerases and integrating the system into metabolic pathways that generate non-natural metabolites for therapeutics.