Isolation of a trans-dominant histidase-negative mutant of Salmonella typhimurium

A mutation of Salmonella typhimurium was obtained that results in the failure of cells to synthesize the enzyme l-histidine ammonia-lyase (histidase). The mutation mapped within the hutH gene and in merodiploid strains was dominant over the wild-type allele. Extracts from cells bearing the trans-dominant histidase-negative allele were shown to contain material that reacts immunologically with antiserum against purified wild-type histidase. It is proposed that the trans-dominant allele results in the synthesis of defective histidase subunits that can combine with, and partially inactivate, wild-type histidase subunits. This subunit mixing presumably does occur, as the enzyme synthesized in a hybrid merodiploid strain is abnormally heat sensitive.