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Graphical Abstract
The substrate specificity of bacterial TGT and Q precursor transporter protein shifts from preQ1 to the queuine base in intracellular human pathogens such as Chlamydia trachomatis, driven by adaptation to the intracellular environment. The facultative intracellular pathogen Bartonella henselae exhibits a rare dual-substrate specificity for these Q salvage enzymes that represents an evolutionary transition state.
Abstract
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 Bartonella henselae, which possess only bTGT and QPTR but lack predicted enzymes for converting preQ1 to Q, would be expected to salvage the queuine (q) base, mirroring the scenario for the obligate intracellular pathogen Chlamydia trachomatis. However, sequence analyses indicate that the substrate-specificity residues of their bTGTs resemble those of enzymes inserting preQ1 rather than q. Intriguingly, MS analyses of tRNA modification profiles in B. henselae reveal trace amounts of preQ1, previously not observed in a natural context. Complementation analysis demonstrates that B. henselae bTGT and QPTR not only utilize preQ1, akin to their Escherichia coli counterparts, but can also process q when provided at elevated concentrations. The experimental and phylogenomic analyses suggest that the Q pathway in B. henselae could represent an evolutionary transition among intracellular pathogens – from ancestors that synthesized Q de novo to a state prioritizing the salvage of q. Another possibility that will require further investigations is that the insertion of preQ1 confers fitness advantages when B. henselae is growing outside a mammalian host.
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Funding
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National Research Foundation of Singapore
- Principle Award Recipient: PeterC. Dedon
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National Institute of Environmental Health Sciences
(Award ES024615)
- Principle Award Recipient: PeterC. Dedon
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National Institute of Environmental Health Sciences
(Award ES026856)
- Principle Award Recipient: PeterC. Dedon
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National Institute of General Medical Sciences
(Award GM070641)
- Principle Award Recipient: Valeriede Crecy-Lagard