An unexpected absence of queuosine modification in the tRNAs of an Escherichia coli B strain

The post-transcriptional processing of tRNAs decorates them with a number of modified bases important for their biological functions. Queuosine, found in the tRNAs with GUN anticodons (Asp, Asn, His, Tyr), is an extensively modified base whose biosynthetic pathway is still unclear. In this study, it was observed that the $tRNA^{Tyr}$ from Escherichia coli B105 (a B strain) migrated faster than that from E. coli CA274 (a K-12 strain) on acid urea gels. The organization of $tRNA^{Tyr}$ genes in E. coli B105 was found to be typical of the B strains. Subsequent analysis of $tRNA^{Tyr}$ and $tRNA^{His}$ from several strains of E. coli on acid urea gels, and modified base analysis of tRNA preparations enriched for $tRNA^{Tyr}$, showed that E. coli B105 lacked queuosine in its tRNAs. However, the lack of queuosine in tRNAs was not a common feature of all E. coli B strains. The tgt and queA genes in B105 were shown to be functional by their ability to complement tgt and queA mutant strains. These observations suggested a block at the step of the biosynthesis of $preQ_1$(or $preQ_0$) in the B105 strain. Interestingly, a multicopy vector harbouring a functional tgt gene was toxic to E. coli B105 but not to CA274. Also, in mixed cultures, E. coli B105 was readily competed out by the CA274 strain. The importance of these observations and this novel strain (E. coli B105) in unravelling the mechanism of $preQ_1$ or $preQ_0$ biosynthesis is discussed