Evolution of Arginine Biosynthesis in the Bacterial Domain: Novel Gene-Enzyme Relationships from Psychrophilic Moritella Strains ( Vibrionaceae ) and Evolutionary Significance of N -α-Acetyl Ornithinase

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Abstract
In the arginine biosynthetic pathway of the vast majority of prokaryotes, the formation of ornithine is catalyzed by an enzyme transferring the acetyl group of N -α-acetylornithine to glutamate (ornithine acetyltransferase [OATase]) ( argJ encoded). Only two exceptions had been reported—the Enterobacteriaceae and Myxococcus xanthus (members of the γ and δ groups of the class Proteobacteria , respectively)—in which ornithine is produced from N -α-acetylornithine by a deacylase, acetylornithinase (AOase) ( argE encoded). We have investigated the gene-enzyme relationship in the arginine regulons of two psychrophilic Moritella strains belonging to the Vibrionaceae , a family phylogenetically related to the Enterobacteriaceae . Most of the arg genes were found to be clustered in one continuous sequence divergently transcribed in two wings, argE and argCBFGH(A) [“ H(A) ” indicates that the argininosuccinase gene consists of a part homologous to known argH sequences and of a 3′ extension able to complement an Escherichia coli mutant deficient in the argA gene, encoding N -α-acetylglutamate synthetase, the first enzyme committed to the pathway]. Phylogenetic evidence suggests that this new clustering pattern arose in an ancestor common to Vibrionaceae and Enterobacteriaceae , where OATase was lost and replaced by a deacylase. The AOase and ornithine carbamoyltransferase of these psychrophilic strains both display distinctly cold-adapted activity profiles, providing the first cold-active examples of such enzymes.