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]) (argJencoded). Only two exceptions had been reported—theEnterobacteriaceae and Myxococcus xanthus(members of the γ and δ groups of the classProteobacteria, 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 theVibrionaceae, a family phylogenetically related to theEnterobacteriaceae. 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 colimutant deficient in the argA gene, encodingN-α-acetylglutamate synthetase, the first enzyme committed to the pathway]. Phylogenetic evidence suggests that this new clustering pattern arose in an ancestor common toVibrionaceae 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.
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