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 tRNA2Glu or both tRNA1Ile and tRNA1BAla genes. In addition, various tRNA genes are located at or near the distal ends of rRNA operons. In particular, genes for tRNATrp and tRNA1Asp 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 tRNATrp 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 tRNATrp and tRNA1Asp at the distal end of rrnC and strongly suggest that they are co-transcribed with the genes for 16S, tRNA2Glu, 23S and 5S RNA. Other such distal tRNAs have been identified, and it is suggested that they too are part of rRNA operons.
Bibliographical noteFunding Information:
We thank A. E. Arfsten and G. D. Strycharz for technical assistance. This work was supported in part by the College of Agriculture and Life Sciences, University of Wisconsin, and by grants from the NIH and the NSF. E. A. M. is the recipient of an NIH postdoctoral fellowship and A. M. F. is the recipient of an American Cancer Society postdoctoral fellowship.