Characterization of the Saccharomyces cerevisiae ARG7 Gene Encoding Ornithine Acetyltransferase, An Enzyme also Endowed with Acetylglutamate Synthase Activity

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Abstract
We have cloned by functional complementation and characterized the yeast ARG7 gene encoding mitochondrial ornithine acetyltransferase, the enzyme catalyzing the fifth step in arginine biosynthesis. While forming ornithine, this enzyme regenerates acetylglutamate, also produced in the first step by the ARG2‐encoded acetylglutamate synthase. Interestingly, total deletion of the genomic ARG7 ORF resulted in an arginine‐leaky phenotype, indicating that yeast cells possess an alternative route for generating ornithine from acetylornithine. Yeast ornithine acetyltransferase has been purified and characterized previously as a heterodimer of two subunits proposed to derive from a single precursor protein [Liu, Y.‐S., Van Heeswijck R., Hoj, P. & Hoogenraad, N. (1995) Eur. J. Biochem. 228, 291–296]; those authors further suggested that the internal processing of Arg7p, which is a mitochondrial enzyme, might occur in the matrix, while the leader peptide would be of the non‐cleavable‐type. The characterization of the gene (a) establishes that Arg7p is indeed encoded by a single gene, (b) demonstrates the existence of a cleaved mitochondrial prepeptide of eight residues, and (c) shows that the predicted internal processing site is unlike the mitochondrial proteolytic peptidase target sequence. Yeast Arg7p shares between 32–43% identity in pairwise comparisons with the ten analogous bacterial ArgJ enzymes characterized. Among these evolutionarily related enzymes, some but not all appear bifunctional, being able to produce acetylglutamate not only from acetylornithine but also from acetyl‐CoA, thus catalyzing the same reaction as the apparently unrelated acetylglutamate synthase. We have addressed the question of the bifunctionality of the eucaryotic enzyme, showing the overexpressed ARG7 can complement yeast arg2 and Escherichia coli argA mutations (affecting acetylglutamate synthase). Furthermore, Arg7p‐linked acetylglutamate synthase activity was measurable in an assay. The yeast enzyme is thus clearly, albeit modestly, bifunctional. As with several bacterial ornithine acetyltransferases, the activity of Arg7p was practically insensitive to arginine but strongly inhibited by ornithine, which behaved as a competitive inhibitor.