L1O Is Rapidly Degraded When Synthesized in Excess of Ribosomal Protein L7/L12

In Escherichia coli the genes encoding ribosomal proteins L10 and L7/12, rpU and rplL, respectively, are cotranscribed and subject to translational coupling. Synthesis of both proteins is coordinately regulated at the translational level by binding of L10 or a complex of L10 and L7/L12 to a single target in the mRNA leader region upstream of rpU. Unexpectedly, small deletions that inactivated the ribosome-binding site of the rpIL gene carried on multicopy plasmids exerted a negative effect on expression of the upstream rpU gene. This effect could be overcome by overproduction of L7/L12 in trans from another plasmid. This apparent stimulation resulted from stabilization of the overproduced L10 protein by L7/L12, presumably because free L10, in contrast to L10 complexed with L7/L12, is subject to rapid proteolytic decay. The contribution of this decay mechanism to the regulation of the rpUL operon is evaluated.