Role of gene overlap in the regulation of mRNA translation for mitochondrial cytochrome P-450c27/25 in the rat.

Previously published results have revealed sequence complementarity between the 5'-terminal regions of mRNAs for hepatic mitochondrial cytochrome P-450c27/ 25 (c27/25) and serine protease inhibitors (SPI) and predicted a role for this sequence overlap in both the regulation of c27/25 mRNA transcription and translation. The possibility that c27/25 mRNA forms an RNA duplex with complementary sequences of SPI mRNAs in vivo was demonstrated in the rat liver and COS-1 cells cotransfected with c27/25 and SPI2.1 plasmids. Quantitative evaluation of RNA duplex in COS-1 cells revealed that most of the c27/25 mRNA exists in duplex form when SPI2.1 mRNA was present at 5-10-fold that of c27/25 mRNA, a ratio comparable to that observed between these two RNAs in the liver. In cotransfected COS-1 cells with the same ratio of mRNAs, highly significant inhibition of the c27/25 mRNA translation (66-75%) was observed, while its transcription remained unaffected. The partial inhibition of c27/25 mRNA translation, even when most of it exists in duplex form, suggests that RNA duplex is undergoing some type of cytoplasmic processing to disengage c27/25 mRNA and make it available for translation. These results imply that abundant endogenous SPI RNAs are able to regulate the c27/25 gene expression.