Protein dosage of the lldPRD operon is correlated with RNase E-dependent mRNA processing

The ability of to grow on L-lactate as a sole carbon source depends on the expression of the operon. A striking feature of this operon is that the transcriptional regulator (LldR) encoding gene is located between the permease (LldP) and the dehydrogenase (LldD) encoding genes. In this study we report that dosage of the LldP, LldR, and LldD proteins is not modulated on the transcriptional level. Instead, modulation of protein dosage is primarily correlated with RNase E-dependent mRNA processing events that take place within the mRNA, leading to the immediate inactivation of , to differential segmental stabilities of the resulting cleavage products, and to differences in the translation efficiencies of the three cistrons. A model for the processing events controlling the molar quantities of the proteins in the operon is presented and discussed. Adjustment of gene expression is critical for proper cell function. For the case of polycistronic transcripts, posttranscriptional regulatory mechanisms can be used to fine-tune the expression of individual cistrons. Here, we elucidate how protein dosage of the operon, which presents the paradox of having the gene encoding a regulator protein located between genes that code for a permease and an enzyme, is regulated. Our results demonstrate that the key event in this regulatory mechanism involves the RNase E-dependent cleavage of the primary transcript at internal site(s) located within the cistron, resulting in a drastic decrease of intact mRNA, to differential segmental stabilities of the resulting cleavage products, and to differences in the translation efficiencies of the three cistrons.