Genetic Analysis Identifies a Function for the queC ( ybaX ) Gene Product at an Initial Step in the Queuosine Biosynthetic Pathway in Escherichia coli

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Abstract
Queuosine (Q), one of the most complex modifications occurring at the wobble position of tRNAs with GUN anticodons, is implicated in a number of biological activities, including accuracy of decoding, virulence, and cellular differentiation. Despite these important implications, its biosynthetic pathway has remained unresolved. Earlier, we observed that a naturally occurring strain of Escherichia coli B105 lacked Q modification in the tRNAs. In the present study, we developed a genetic screen to map the defect in E. coli B105 to a single gene, queC (renamed from ybaX ), predicted to code for a 231-amino-acid-long protein with a pI of 5.6. As analyzed by mobility of tRNA Tyr on acid urea gels and two-dimensional thin-layer chromatography of the modified nucleosides, expression of QueC from a plasmid-borne copy confers a Q + phenotype to E. coli B105. Further, analyses of tRNA Tyr from E. coli JE10651 ( queA mutant), its derivative generated by deletion of chromosomal queC ( queA Δ queC ), and E. coli JE7325, deficient in converting preQ 0 to preQ 1 , have provided the first genetic evidence for the involvement of QueC at a step leading to production of preQ 0 , the first known intermediate in the generally accepted pathway that utilizes GTP as the starting molecule. In addition, we discuss the possibilities of collaboration of QueC with other cellular proteins in the production of preQ 0 .