The Primary Structure of the Imported Mitochondrial Protein, Ornithine Transcarbamylase From Rat Liver: mRNA Levels During Ontogeny

Abstract

Ornithine transcarbamylase, one of the enzymes of the urea cycle in ureotelic organisms, is synthesized in the cytoplasm of hepatocytes as a precursor larger than the mature form found in the mitochondrial matrix. The amino acid sequence of the precursor of ornithine transcarbamylase was deduced from rat liver from the nucleotide sequence of overlapping complementary DNA clones spanning the complete coding region, 3'' untranslated region and most of the 5'' untranslated region of the mRNA. The mature enzyme consists of 322 amino acids and is derived from the largest precursor by proteolytic removal of 32 amino acids from the amino-terminus. The amino-terminal extension contains 8 basic and no acidic residues. This highly basic character appears to be a feature of presequences on cytoplasmically synthesized mitochondrial proteins. Comparison of the amino acid sequence determined for the enzyme from rat with that from human liver shows that there si a high degree of homology between the sequence of the mature protein (93%) and relatively less homology between the sequences of the amino-terminal extension (72%). The ornithine transcarbamylase from rat liver also shows a considerable degree of amino acid homology (44%) with the enzyme from Escherichia coli and leads t suggestions about residues involved in substrate binding and catalysis. An analysis of levels of RNA in fetal and neonatal liver shows that ornithine transcarbamylase mRNA levels increase from .apprx. 40% of adult levels at day 14 of gestation of a peak at day 20 of gestation, and, after a drop around the time of birth, rises to adult levels during the 2nd wk after birth.