Structures, Expression and their Desorders of the Genes for Mitochondrial Proteins

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1. The rat mitochondrial ornithine carbamoyltransferase (EC 2.1.3.3) gene located on the X chromosome and expressed specifically in the liver and small intestine was cloned and the structure determined. This gene is 75 kilobases long and is split into ten exons. The introns range from 85 bases to 26 kilobases. The sum of the total exons is 1.5 kilobases and occupies only 2% of the gene; this value being one of the lowest among genes heretofore reported. The first exon encodes most of the NH2- Terminal presequence which functions as a mitochondrial targeting signal. Putative binding sites for the two substrates of the enzyme, carbamoyl phosphate and ornithine, are encoded by exon 3 and 9, respectively. A set of "CAAT" and "ATA" box-like sequences is present around the position 200 bases upstream from the 5' end of the mRNA. About 35 bases downstream from this set of putative promoter elements, an 11-nucleotide sequence around the 5' end of the mRNA reappears, as a direct repeat. Upstrea … More m or downstream from the 5' end of the mRNA there are several noteworthy sequences which resemble the Sp1-binding site, the enhancer core sequence, the consensus sequence for the glucocorticoid receptor binding sites, and the putative enhancer element of another gane which is expressed specifically in the liver.2. Unlike most mitochondrial matrix proteins, mitochondrial 3-oxoacyl-CoA thiolase in rats is synthesized with no transient presequence and possesses information for mitochondrial targeting and import in the mature protein. Two overlapping cDNA clones contained an open reading frame encoking a polypeptide of 397 amino acid residues (predicted Mr=41868), 5' untranslated sequence of 164 base pairs, 3' untranslated sequence of 264 base pairs and poly(A) tract. The amino acid sequence of the mitochondrial thiolase is 37% identical with that of the mature portion of rat peroxisomal 3-oxoacyl- CoA thiolase precursor. These results suggest that the two thiolases have a common origin and obtained information for targeting to respective organelles during evolution. Two sequences in the mitochondrial thiolase that may serve as a mitochondrial targeting signal are presented. Less