Post-transcriptional control of coordinated ribosomal protein synthesis in Escherichia coli

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Abstract
The synthesis of the approximately 50 different ribosomal proteins (r proteins) is very well coordinated1 in Escherichia coli. As the r-protein genes are arranged in many different operons, placed at separate locations on the E. coli chromosome2–5, it is not obvious how this coordination is maintained. The first indication that special controls are involved in the coordination came from the observations that merodiploid strains containing F′ factors with some of the ribosomal genes also synthesise the r proteins in balanced amounts6,7. The overall regulation of r-protein synthesis in response to changes in growth conditions is primarily mediated by changes in the rate of transcription of the r-protein genes8. To investigate whether the gene dosage control seen in merodiploid strains is also transcriptional in nature or whether other mechanisms are involved, we compared the transcription of r-protein mRNA in haploid and merodiploid strains. It was found that the rate of transcription of the r-protein mRNA from the str-spc cluster of genes changes in proportion to the gene dosage and it is concluded that the expression of r-protein genes is adjusted by post-transcriptional control.