Queuosine Salvage inBartonella henselaeHouston 1: A Unique Evolutionary Path
Authors: Quaiyum Samia , Yuan Yifeng , Sun Guangxin , Ratnayake R. M. Madhushi N. , Hutinet Geoffrey , Dedon Peter C. , Minnick Michael F. , de Crécy-Lagard Valérie 
Published: December 2023

Queuosine (Q) stands out as the sole tRNA modification that can be synthesized via salvage pathways. Comparative genomic analyses identified specific bacteria that showed a discrepancy between the projected Q salvage route and the predicted substrate specificities of the two identified salvage proteins: 1) the distinctive enzyme tRNA guanine-34 transglycosylase (bacterial TGT, or bTGT), responsible for inserting precursor bases into target tRNAs; and 2) Queuosine Precursor Transporter (QPTR), a transporter protein that imports Q precursors. Organisms like the facultative intracellular pathogenBartonella henselae, which possess only bTGT and QPTR but lack predicted enzymes for converting preQ1to Q, would be expected to salvage the queuine (q) base, mirroring the scenario for the obligate intracellular pathogenChlamydia trachomatis. However, sequence analyses indicate that the substrate-specificity residues of their bTGTs resemble those of enzymes inserting preQ1rather than q. Intriguingly, mass spectrometry analyses of tRNA modification profiles inB. henselaereveal trace amounts of preQ1, previously not observed in a natural context. Complementation analysis demonstrates thatB. henselaebTGT and QPTR not only utilize preQ1, akin to theirEscherichia colicounterparts, but can also process q when provided at elevated concentrations. The experimental and phylogenomic analyses suggest that the Q pathway inB. henselaecould represent an evolutionary transition among intracellular pathogens—from ancestors that synthesized Qde novoto a state prioritizing the salvage of q. Another possibility that will require further investigations is that the insertion of preQ1has fitness advantages whenB. henselaeis growing outside a mammalian host.

Author summary

Transfer RNAs (tRNAs) are adaptors that deliver amino acids to ribosomes during translation of messenger RNAs (mRNAs) into proteins. tRNA molecules contain specially-modified nucleotides that affect many aspects of translation, including regulation of translational efficiency, as modified nucleotides primarily occur near the portion of tRNA (anticodon) that directly interacts with the coding sequence (codon) of the mRNA while it is associated with a ribosome. Queuosine (Q) is a modified tRNA nucleotide located in the anticodon that can be synthesized or uniquely imported from the environment as Q or a precursor using a salvage mechanism. Free-living bacteria, e.g.,E. coli, can synthesize Q or salvage precursors from the environment, but many obligate intracellular pathogens, e.g.,Chlamydia trachomatis, cannot synthesize Q and must import a precursor from eukaryotic hosts. In this study, we determined thatBartonella henselae, a facultative intracellular bacterial pathogen of vascular cells, falls somewhere in the middle, as it is unable to synthesize Q but can salvage Q or certain precursors. The unusual nature ofBartonella’s system suggests different evolutionary scenarios. It could be a snapshot of the transition from Q synthesis to strict Q salvage or represent a unique adaptation to a complex multi-host lifestyle.