Evolution of glutamate acetylation from a primordial N- acetyltransferase. The recruitment of a primordial N-acetyltransferase for L-glutamate acetylation in the first step of arginine biosynthesis has been made according to at least three different evolutionary ways. The events of gene duplication and gene fusion that allowed evolution toward either the two-domain N-acetylglutamate synthase (EC 18.104.22.168) or the bifunctional argH(A) fusion are schematized. This scheme does not specify whether the argH(A) fusion arose in an organism originally devoid of NAGS or in an organism having lost NAGS. The yellow domains in contemporary proteins (bottom line) are bearing the ArgA activity (EC 22.214.171.124). The contemporary proteins that have been experimentally studied are indicated between brackets.