Physical and Chemical Characterization of the Isomerase of Histidine Biosynthesis in Salmonella typhimurium

The amino acid composition and chemical and immunological properties of the isomerase catalyzing the fourth step in histidine biosynthesis in Salmonella typhimurium are reported. The molecular weight of the native enzyme is 29,000, as determined by high speed equilibrium centrifugation; it is unchanged after complete reduction and alkylation, indicating the presence of a single polypeptide chain. This conclusion is supported by the finding of a single carboxyl-terminal amino acid, valine, both by digestion with carboxypeptidase A and by hydrazinolysis. Furthermore, the number of spots seen on peptide maps of tryptic digests of carboxymethylated isomerase is approximately the same as the number of arginine plus lysine residues found by amino acid analyses. These results agree with results of genetic studies (1) which show that the structural gene for this enzyme consists of a single complementation group. The minor active isomers of the enzyme, previously described, appear to result from modifications in vitro of a single protein. Antibody specific to the isomerase has been prepared. Carboxymethylated isomerase and isomerase derived from extracts of repressed and derepressed histidine auxotrophs with mutations in all regions of the histidine operon, of constitutive mutants, and of the wild type strains LT-2 and LT-7 are antigenically identical.