Genetically determined differences in concentrations of isoaccepting tRNAs in Escherichia coli

Threonine tRNAs and their genes in Escherichia coli

The subject of this study was the threonine isoacceptor family of tRNAs in Escherichia coli and the genes coding for them. The goal was to identify and map all the genes and to determine the relative contribution of each gene to the tRNA pool. The mapping experiments exploited gene-dosage effects in partially diploid strains; if a strain harboring a particular F' episome overproduced a particular tRNA species, it could be concluded that the gene for that tRNA was located on the chromosomal segment carried by the F'. Isoacceptor tRNAs were distinguished by column fractionation. It was found that there are three major threonine tRNA species that occur in roughly equal amounts. These are tRNAThr1, which is encoded by a gene in the distal region of the rrnD ribosomal RNA operon, and tRNAThr3 and tRNAThr4, which comes from genes in the cluster thrU tyrU glyT thrT at 89 min on the map. The relative abundances of the tRNA species roughly match the reported frequencies of the codons that they recognize in mRNA. Although the tRNAThr4 has a mismatched base pair that raised questions about its biological activity, it was found to be functional at least with respect to recognition by the threonyl-tRNA synthetase. An apparent fourth gene affecting threonine tRNA has been identified and mapped at 6-8 min; it is here designated thrW. It may be a structural gene for a minor tRNA species, present in one-third the amount of each of the major species, and chromatographically indistinguishable from tRNAThr4.

Genetic analysis of mutations causing borrelidin resistance by overproduction of threonyl-transfer ribonucleic acid synthetase

Mutations leading to borrelidin resistance in Escherichia coli by overproduction of threonyl-transfer ribonucleic acid synthetase were anaylzed genetically. The regulatory mutations were closely linked to the treonyl-transfer ribonucleic acid synthetase structural gene (thrS), located clockwise to it. The mutation that causes the threefold-increased enzyme level was more distant from thrS than the mutation responsible for the ninefold overproduction. Both mutations were cis dominant in merodiploid strains, indicating that they affected promoter-operator-like control elements. Overproduction was restricted to threonyl-transfer ribonucleic acid synthetase and was not observed for the products of genes neighboring thrS (e.g., infC, pheS, pheT, and argS), providing evidence that thrS is transcribed singly and that gene amplificationis not a likely basis for increased thrS experession.