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 asp artate semialdehyde and pyruvate have been identified in a number of bacter ial genera, with the single exception of the dapC gene encoding the PLP-dep endent N-succinyl-L,L-diaminopimelate:alpha-ketoglutarate aminotransferase (DapATase). Purification of E. coli DapATase allowed the determination of b oth the aminoterminal 26 amino acids and a tryptic peptide fragment. Sequen ce analysis identified both of these sequences as being identical to corres ponding sequences from the PLP-dependent E. coli argD-encoded N-acetylornit hine aminotransferase (NAcOATase). This enzyme performs a similar reaction to that of DapATase, catalyzing the N-acetylornithine-dependent transaminat ion of alpha-ketoglutarate. PCR cloning of the argD gene from genomic E. co li DNA, expression, and purification yielded homogeneous E. coli NAcOATase. This enzyme exhibits both NAcOATase and DapATase activity, with similar sp ecificity constants for N-acetylornithine and N-succinyl-L,L-DAP, suggestin g that it can function in both lysine and arginine biosynthesis. This findi ng may explain why numerous investigations have failed to identify genetica lly the bacterial dapC locus, and suggests that this enzyme may be an attra ctive target for antibacterial inhibitor design due to the essential roles of these two pathways in bacteria.