Regulation of ribosomal protein synthesis in Escherichia coli by selective mRNA inactivation

In an Escherichia coli strain lysogenic for lambda spc2 transducing phage, an extra copy of ribosomal protein (r-protein) genes in the spc and alpha operons are carried on the phage chromosome. Expression of genes in the spc operon in this merodiploid strain was compared with that in a control "haploid" strain carrying lambda trkA phage. It was found that the synthesis rate of spc mRNA, relative to other reference mRNA in the merodiploid strain, is about 2-fold higher than that in the control strain; yet, no dosage effect was observed in the synthesis rate of r-proteins in the spc or alpha operon. The spc mRNA was found to be more rapidly degraded in the merodiploid strain than in the control strain, and its steady-state amount, relative to reference mRNA, was only slightly higher in the merodiploid strain than in the control strain. Thus, E. coli cells have the ability to regulate the rate of r-protein synthesis regardless of the rate of transcription of r-protein genes, presumably by inactivation of the mRNA followed by its degradation. A model is proposed which involves selective inactivation of r-protein mRNA by a feedback mechanism. The model can explain coordinated synthesis of r-proteins and other observations related to selective expression of certain alleles in diploid strains.