Regulation of the Expression of COI and COIII mRNAs in Rat Liver Mitochondria
Recent work in several laboratories has produced a great deal of information on the structure of mitochondrial (mt) genomes of some mammalian species. MtDNA sequence of man, cow and mouse as well as large part of rat has been determined and the transcription products of these DNAs have been identified and mapped1,2. Mammalian mtDNA codes for 2 rRNAs, 11 mRNAs specifying 13 polypeptides for respiratory complexes and 22 tRNAs. Despite the complete transcription of both strands, the location of the structural genes is asymmetrical: the L-strand codes only for ND6 mRNA and eight tRNAs, while the rest of the genes is coded for by the H-strand. The tRNA genes are interspersed with almost absolute regularity among the rRNA and the mRNA coding sequences; this arrangement is consistent with a RNA processing mechanism in which the cloverleaf structure of the tRNA genes is used as a recognition signal for RNAse P-like enzymes involved in mtRNA processing. The gene organization of rat liver mtDNA is reported in Fig.1. The mammalian mt transcription starts from three initiation sites, one placed on the L-strand and two on the H-strand: this means that while the L-strand contains only one transcription unit, the H-strand is transcribed according to two different modes. Data obtained mainly in human cells3,4 have generated an H-strand transcription mechanism which implies that one transcription unit initiates 16 nt upstream of the tRNAPhe gene, giving rise to the two rRNAs 16S and 12S and to the tRNAPhe and tRNAVal. The other transcriptional event starts in proximity of the 5′ end of the 12S rRNA; it originates a polycistronic transcript that is processed in correspondence of the tRNA genes.
KeywordsHeLa Cell tRNA Gene Transcription Unit BeLa Cell Mitochondrial Gene Expression
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