Queuosine salvage in Houston 1: a unique evolutionary path.

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 such as the facultative intracellular pathogen , which possess only bTGT and QPTR but lack predicted enzymes for converting preQ to Q, would be expected to salvage the queuine (q) base, mirroring the scenario for the obligate intracellular pathogen . However, sequence analyses indicate that the substrate-specificity residues of their bTGTs resemble those of enzymes inserting preQ rather than q. Intriguingly, MS analyses of tRNA modification profiles in reveal trace amounts of preQ, previously not observed in a natural context. Complementation analysis demonstrates that bTGT and QPTR not only utilize preQ, akin to their counterparts, but can also process q when provided at elevated concentrations. The experimental and phylogenomic analyses suggest that the Q pathway in could represent an evolutionary transition among intracellular pathogens - from ancestors that synthesized Q to a state prioritizing the salvage of q. Another possibility that will require further investigations is that the insertion of preQ confers fitness advantages when is growing outside a mammalian host.