New Enzyme AvaS Builds Aminovaleramide on tRNA Using Vitamin B6 Chemistry

A newly characterized biosynthetic pathway has revealed how bacteria can install an unusual amide-containing modification onto transfer RNA using one of biology’s most versatile cofactors. The enzyme AvaS, described in Nature Chemical Biology, uses pyridoxal phosphate, the chemically active form of vitamin B6, to construct aminovaleramide and attach it to a tRNA substrate, expanding the known chemical repertoire of RNA modification enzymes. The finding matters because tRNA modifications sit at the heart of how cells translate genetic information accurately, and every newly discovered route to these chemical decorations deepens understanding of both fundamental metabolism and the evolutionary ingenuity of microbes.

Transfer RNA molecules are not naked RNA strands; they are heavily decorated with chemical groups that fine-tune their stability, folding, and ability to recognize codons on the ribosome. More than a hundred distinct modified nucleosides have been catalogued across the three domains of life, ranging from simple methyl groups to elaborate, multi-ring structures that require whole cascades of enzymes to assemble. Some of the most chemically ambitious modifications are found in bacteria, where they help organisms survive in hostile environments, tune translation in response to stress, and even contribute to antibiotic resistance. The aminovaleramide modification reported by the AvaS research joins this growing catalogue as a striking example of amide chemistry carried out directly on RNA.

Pyridoxal phosphate, commonly abbreviated PLP, is a cofactor with a legendary reputation in enzymology. It sits at the reactive heart of enzymes that make, break, and rearrange amino acids, enabling transformations that would otherwise demand harsh laboratory conditions. PLP achieves this by forming an internal aldimine, a Schiff base linkage, with a lysine residue in the enzyme’s active site. When an amino acid substrate arrives, the linkage is exchanged in a transamination step that produces an external aldimine, positioning the substrate for reactions ranging from decarboxylation to side-chain cleavage. What makes the AvaS discovery remarkable is that this canonical amino acid chemistry appears to have been recruited for RNA biosynthesis, in effect coupling amino acid metabolism to the chemical maturation of tRNA.

According to the study, AvaS catalyzes the formation of aminovaleramide through a PLP-dependent route that resembles pathways used to synthesize certain amino acid-derived metabolites. The reaction logic involves the cofactor-mediated processing of an amino acid precursor, likely through condensation and rearrangement steps that generate an activated intermediate, followed by transfer of the resulting aminovaleramide moiety onto the tRNA scaffold. This kind of cofactor-dependent construction on RNA is rare. Most known tRNA modification enzymes rely on S-adenosylmethionine for methyl and threonylcarbamoyl chemistry, or on ATP-driven activation reactions to ligate smaller groups onto nucleotides. A PLP enzyme acting on tRNA therefore represents a mechanistic surprise, suggesting that the boundary between primary metabolism and RNA modification biochemistry is more porous than previously appreciated.

The identification of AvaS also illustrates the power of modern bioinformatic screens. Rather than stumbling across the enzyme by chance, the researchers were able to trace the modification’s occurrence by following the distribution of gene clusters whose sequence features hinted at PLP-dependent chemistry linked to RNA processing. Genes encoding tRNA modification enzymes frequently cluster with partner genes, exporter proteins, or resistance determinants, a pattern known as neighboring gene logic. By scanning bacterial genomes for such clusters, and by asking which organisms contain both the predicted modification machinery and the chemical signature of aminovaleramide-containing nucleosides, the team narrowed the search to a manageable set of candidate enzymes and then validated their predictions experimentally.

Once AvaS was confirmed as the biosynthetic enzyme, the structural and mechanistic characterization proceeded along classical enzymological lines, with modern tools. Recombinant production of the protein allowed the researchers to test its activity in vitro, demonstrating that purified AvaS could carry out the key chemical steps without the rest of the cellular milieu. Mass spectrometry of digested nucleosides confirmed the identity of the aminovaleramide product, while comparisons with catalytic mutants and cofactor-free controls established that PLP is genuinely required, not merely tolerated. These experiments collectively argue that a single enzyme can perform a multi-step biosynthesis, assembling the modification before or during its attachment to RNA, rather than relying on a separate pathway to pre-build the transferable group.

Why would a bacterium invest energy in constructing such an elaborate modification? The most likely answers relate to translation fidelity and stress physiology. Modified bases near the anticodon loop of tRNA influence how reliably the molecule pairs with messenger RNA codons, and disruptions to these modifications typically cause ribosomal frameshifting, slowed growth, or heightened sensitivity to environmental challenges. Amide-bearing modifications can also alter the local geometry and hydrogen-bonding pattern of the tRNA in ways that stabilize particular conformations of the anticodon loop. In pathogenic or environmental bacteria, such fine-tuning can mean the difference between thriving and failing under thermal, oxidative, or nutrient stress, which in turn makes the underlying enzymes attractive subjects for study as potential antimicrobial targets.

The discovery carries implications beyond microbiology. RNA chemical biology has been undergoing a renaissance, driven partly by interest in modified nucleosides as biomarkers, as regulators of gene expression, and as engineering targets for synthetic biology. Finding that a cofactor as central as PLP participates directly in RNA modification suggests that other seemingly improbable chemistries may also be lurking in unexplored genomic corners. The aminovaleramide modification itself, featuring a terminal amino group on a five-carbon chain linked through an amide bond, is chemically rich, and understanding how enzymes build and install such groups could inspire new methods for site-specific RNA labeling or the design of modified oligonucleotide therapeutics.

There are also evolutionary questions raised by the work. PLP-dependent enzymes form large and ancient protein superfamilies, and the AvaS result adds a new functional branch to that family tree. Determining whether the RNA-modifying activity arose by divergence from an amino acid biosynthetic ancestor, or through convergent recruitment of PLP chemistry into an unrelated scaffold, will require broader phylogenetic analysis. The study’s genomic survey provides a starting point, mapping where AvaS homologs occur across bacterial phyla and hinting at horizontal gene transfer events that may have spread the capability between distantly related organisms. Such analyses often reveal that RNA modification systems evolve rapidly, shaped by the arms races between microbes, their viruses, and their chemical environments.

For the field of tRNA biology, the AvaS report is a reminder of how much chemical diversity remains undocumented. Decades of focused work on well-studied model organisms, such as Escherichia coli and Saccharomyces cerevisiae, produced detailed maps of their modification landscapes, but the vast majority of bacterial species have never been surveyed with modern nucleoside mass spectrometry. As high-throughput analytical methods and genome-mining approaches converge, enzymes like AvaS are expected to surface with increasing frequency, each one a potential new tool for manipulating RNA and a potential window into unexplored metabolic logic. The PLP-dependent construction of aminovaleramide stands as an early and vivid example of what that exploration is likely to yield.

Subject of Research: Pyridoxal phosphate-dependent biosynthesis of the tRNA modification aminovaleramide by the bacterial enzyme AvaS

Article Title: Pyridoxal-phosphate-dependent biosynthesis of aminovaleramide by AvaS in tRNA

Article References: Sun, J., Wu, J., Yuan, Y., Balamkundu, S., Dziergowska, A., Chay Suen Suen, H., Dwijapriya, Liang, C., Hardy, L., Lee, M. E., Leszczynska, G., Liu, C.-F., Baharoglu, Z., Drouard, L., Bruner, S. D., Begley, T. J., de Crécy-Lagard, V., & Dedon, P. C. (2026). Pyridoxal-phosphate-dependent biosynthesis of aminovaleramide by AvaS in tRNA. Nature Chemical Biology. https://doi.org/10.1038/s41589-026-02303-0

Image Credits: AI Generated

DOI: 10.1038/s41589-026-02303-0

Keywords: tRNA modification, AvaS, pyridoxal phosphate, aminovaleramide, vitamin B6, enzyme mechanism, RNA chemical biology, bacterial genomes, translation fidelity, nucleoside mass spectrometry, bioinformatic gene mining, Nature Chemical Biology