The Escherichia coli COG1738 Member YhhQ Is Involved in 7-Cyanodeazaguanine (preQ(0)) Transport

Queuosine (Q) is a complex modification of the wobble base in tRNAs with GUN anticodons. The full Q biosynthesis pathway has been elucidated in Escherichia coli. FolE, QueD, QueE and QueC are involved in the conversion of guanosine triphosphate (GTP) to 7-cyano-7-deazaguanine (preQ(0)), an intermediate of increasing interest for its central role in tRNA and DNA modification and secondary metabolism. QueF then reduces preQ(0) to 7-aminomethyl-7-deazaguanine (preQ(1)). PreQ(1) is inserted into tRNAs by tRNA guanine((34)) transglycosylase (TGT). The inserted base preQ(1) is finally matured to Q by two additional steps involving QueA and QueG or QueH. Most Eubacteria harbor the full set of Q synthesis genes and are predicted to synthesize Q de novo. However, some bacteria only encode enzymes involved in the second half of the pathway downstream of preQ(0) synthesis, including the signature enzyme TGT. Different patterns of distribution of the queF, tgt, queA and queG or queH genes are observed, suggesting preQ(0), preQ(1) or even the queuine base being salvaged in specific organisms. Such salvage pathways require the existence of specific 7-deazapurine transporters that have yet to be identified. The COG1738 family was identified as a candidate for a missing preQ(0)/preQ(1) transporter in prokaryotes, by comparative genomics analyses. The existence of Q precursor salvage was confirmed for the first time in bacteria, in vivo, through an indirect assay. The involvement of the COG1738 in salvage of a Q precursor was experimentally validated in Escherichia coli, where it was shown that the COG1738 family member YhhQ is essential for preQ(0) transport.