Expression of rRNA and tRNA genes in Escherichia coli: evidence for feedback regulation by products of rRNA operons.

We have tested a model for global ribosome biosynthesis by examining the effects of increased gene dosage on the synthesis of rRNA. Increasing gene dosage does not lead to a significant increase in total rRNA transcription; i.e., rRNA synthesis from individual rRNA operons is reduced to keep total rRNA production unchanged. In contrast, when the plasmid-encoded rRNA operons used to increase gene dosage contain deletions within the rRNA coding region, rRNA transcription is gene-dosage-dependent; i.e., rRNA regulation is relieved. We find that the syntheses of most, if not all, tRNAs are also subject to the same controls as rRNA transcription. We conclude that the production of functional rRNA is monitored by the regulatory system that controls rRNA and tRNA transcription. We propose that rRNA and tRNA are negatively controlled by products of rRNA operons and discuss evidence suggesting that ribosomes are the key element involved in the postulated feedback regulation.