Post-transcriptional regulation of the str operon in Escherichia coli. Ribosomal protein S7 inhibits coupled translation of S7 but not its independent translation.
The str operon of Escherichia coli consists of the genes for ribosomal proteins S12 (rpsL) and S7 (rpsG) and elongation factors G (fusA) and Tu (tufA). Previous studies have shown that S7 is a translational feedback repressor and inhibits the synthesis of itself and of elongation factor G. We have now shown that induction of S7 synthesis from the S7 gene fused to the arabinose promoter on a plasmid also leads to inhibition of the synthesis of S12 from the chromosomal S12 gene, and that this regulation takes place using the same target site as that used for distal gene regulation, i.e. S7 retroregulates S12. We have then demonstrated that S7 synthesis is mostly translationally coupled with the translation of the preceding S12 gene. Using a rpsG'-'lacZ fusion gene as a reporter for S7 synthesis, we found that abolishing S12 translation by a mutational alteration of the AUG start codon of the S12 gene leads to about tenfold reduction of S7 synthesis without significantly affecting its rate of transcription. Deletion of the proximal portion of the S12 gene or a premature termination of S12 translation by an amber mutation at the 26th codon also led to a large reduction of S7 synthesis. Unexpectedly, we have discovered that overproduction of S7 in trans from a plasmid leads to repression of the rpsG'-'lacZ fusion gene when the fusion gene is preceded by the intact S12 gene, but not when the S12 gene carried the above-mentioned mutations that abolish S12 translation. Thus, a novel feature of this regulatory system is that translation of S7 achieved by independent initiation is not inhibited by S7 in vivo, whereas translation of S7 achieved by translational coupling is sensitive to S7 repression. These observations also suggest that the coupled S7 translation is probably achieved by the use of ribosomal subunits employed for translation of the upstream S12 gene.