Some rRNA operons in E. coli have tRNA genes at their distal ends

Abstract

We have previously isolated seven rRNA operons on plasmids or λ transducing phages and identified various tRNAs encoded by these operons. Each of the seven operons has one of two different spacer tRNA gene arrangements between the genes for 16S and 23S rRNA: either tRNA 2 Glu or both tRNA 1 Ile and tRNA 1B Ala genes. In addition, various tRNA genes are located at or near the distal ends of rRNA operons. In particular, genes for tRNA Trp and tRNA 1 Asp are located at the distal end of rrnC at 83 min on the E. coli chromosome. Experiments with various hybrid plasmids, some of which lack the rRNA promoter, have now demonstrated that this promoter is necessary for expression of the distal tRNA genes. Rifampicin run-out experiments have also provided evidence that the tRNA Trp gene is located farther from its promoter than the spacer tRNA gene or the 5S RNA gene. These results confirm the localization of genes for tRNA Trp and tRNA 1 Asp at the distal end of rrnC and strongly suggest that they are co-transcribed with the genes for 16S, tRNA 2 Glu, 23S and 5S RNA. Other such distal tRNAs have been identified, and it is suggested that they too are part of rRNA operons.