Evolution of arginine biosynthesis in the bacterial domain: Novel gene-enzyme relationships from psychrophilic Moritella strains (Vibrionaceae) and evolutionary significance of N-alpha-acetyl ornithinase

In the arginine biosynthetic pathway of the vast majority of prokaryotes, t he formation of ornithine is catalyzed by an enzyme transferring the acetyl group of N-alpha-acetylornithine to glutamate (ornithine acetyltransferase [OATase]) (argJ encoded). Only two exceptions had been reported-the Entero bacteriaceae and Myxococcus xanthus (members of the gamma and delta groups of the class Proteobacteria, respectively)-in which ornithine is produced f rom N-a-acetylornithine by a deacylase, acetylornithinase (AOase) (argE enc oded). We have investigated the gene-enzyme relationship in the arginine re gulons of two psychrophilic Moritella strains belonging to the Vibrionaceae , a family phylogenetically related to the Enterobacteriaceae. Mst of the a rg genes were found to be clustered in one continuous sequence divergently transcribed in two wings, argE and argCBFGH(A) ["H(A)" indicates that the a rgininosuccinase gene consists of a part homologous to known argH sequences and of a 3' extension able to complement an Escherichia coli mutant defici ent in the argA gene, encoding N-alpha-acetylglutamate synthetase, the firs t enzyme committed to the pathway]. Phylogenetic evidence suggests that thi s new clustering pattern arose in an ancestor common to Vibrionaceae and En terobacteriaceae, where OATase was lost and replaced by a deacylase. The AO ase and ornithine carbamoyltransferase of these psychrophilic strains both display distinctly cold-adapted activity profiles, providing the first cold -active examples of such enzymes.