Correlation between mutation type and the production of cross-reacting material in mutants of the A gene of the histidine operon in Salmonella typhimurium.

This study concerns the correlation between the type of mutation in a bacterial gene and the ability of the mutant organism to produce immunologically cross-reacting material (CRM). Ninety-five mutants of the A gene of the histidine operon in Salmonella typhimurium have been separated into three classes, by means of mutagenesis and suppression tests, according to the classification scheme devised by H. J. Whitfield, Jr., et al.: (i) missense mutants (in which there is a single amino acid substitution in the A protein); (ii) nonsense mutants (in which various portions of the carboxyl-terminal segment of the A protein are missing); and (iii) deletion and frameshift mutants. Extracts of all mutants were also examined for the presence of CRM. At least 70% of the missense mutants produced CRM. The genetic loci affected in these mutants were randomly distributed throughout the A gene. None of the nonsense, deletion, or frameshift mutants produced CRM. The absence of CRM in the nonsense mutants is in keeping with the finding that removal of the carboxyl-terminal valine residue from the A protein by carboxypeptidase A eliminates antigenicity.