Identification of the argD-encoded n-acetylornithine aminotransferase as n-succinyldiaminopimelate transaminase

The bacterial biosynthesis of L-lysine is catalyzed by sewm enzymes, encoded bv the dapA-I and lysA genes. Uniquely amongst these genes, the dapC gene, encoding N snccinyldiaminopimelate transaminase (SI)PT), has never been cloned or sequenced from any bacterial species. We have purified (SI)PT) from E.coli to near homogeneity, and performed amino terminal and internal amino acid sequencing. The determined amino acid sequences were identical to lhe predicted sequences of lhe argD-encoded N-acetylornithine aminotransferase. We have cloned, expressed and purified the E. colt awD gene product 1o homogeneity, and have confirmed that the gene product catalyzes the ketoglutarale dependenl l ransamination of N-acetylornithine. This enzyme also catalyzes the glutamate-tit, pendent transamination of N-succinyl-2-amino-6-oxo-pimelic acid to generate N succinyl-L,L-diaminopimelate. The V/K values for {he two reactions are within an order of magnitude of each other, suggesting that lhe awD-encoded transaminase may fulfill two catalytically' importanl trausamination reactions in both arginine and lysine biosynthesis. The kinetic and mechanistic characterization of t his bifunctional enz.vme will be reported.