The Dual Biosynthetic Capability of N-Acetylornithine Aminotransferase in Arginine and Lysine Biosynthesis

The genes encoding the seven enzymes needed to synthesize L-lysine from aspartate semialdehyde and pyruvate have been identified in a number of bacterial genera, with the single exception of the dapC gene encoding the PLP-dependent N-succinyl-L, L-diaminopimelate:alpha-ketoglutarate aminotransferase (DapATase). Purification of E. coli DapATase allowed the determination of both the amino-terminal 26 amino acids and a tryptic peptide fragment. Sequence analysis identified both of these sequences as being identical to corresponding sequences from the PLP-dependent E. coli argD-encoded N-acetylornithine aminotransferase (NAcOATase). This enzyme performs a similar reaction to that of DapATase, catalyzing the N-acetylornithine-dependent transamination of alpha-ketoglutarate. PCR cloning of the argD gene from genomic E. coli DNA, expression, and purification yielded homogeneous E. coli NAcOATase. This enzyme exhibits both NAcOATase and DapATase activity, with similar specificity constants for N-acetylornithine and N-succinyl-L,L-DAP, suggesting that it can function in both lysine and arginine biosynthesis. This finding may explain why numerous investigations have failed to identify genetically the bacterial dapC locus, and suggests that this enzyme may be an attractive target for antibacterial inhibitor design due to the essential roles of these two pathways in bacteria.