Isolation of super-repressor mutants in the histidine utilization system of Salmonella typhimurium

Abstract

Two super-repressor mutations in the histidine utilization (hut) operons of Salmonella typhimurium are described. Cells bearing either of these mutations have levels of hut enzymes that do not increase above the uninduced levels when growth is in the presence of either histidine or the gratuitous inducer imidazolepropionate. Both mutations lie in the region of the gene for the hut repressor, hutC, and reverse mutations of both are to the constitutive (repressor-negative) rather than to the inducible (wild type) phenotype. In hybrid merodiploid strains the super-repressor mutations are dominant over either wild-type (hutC+) or repressor-negative (hutC-) alleles. Whereas both super-repressor mutations cause the uninducible synthesis of hut enzymes, the degree of repression is different. One mutation causes repression of enzyme synthesis in one of the two hut operons to a level below the basal, uninduced level of wild-type cells. The other mutation causes repression to a lesser degree than in wild-type cells, so that the hut enzymes are present at a level above the normal basal level; this partially constitutive synthesis is greater for the enzymes of one of the hut operons than for the enzymes of the other. Thus, both mutations apparently result in repressors with altered operator-binding properties, in addition to altered inducer-binding properties.

Involvement of the product of the glnF gene in the autogenous regulation of glutamine synthetase formation in Klebsiella aerogenes

“…Histidine is converted by histidase and urocanase to imidazolonepropionate. This compound is toxic to bacteria carrying a mutation in the hutI gene that prevents its further metabolism; consequently, strains carrying the hutI mutation fail to grow in the presence of 2% histidine at 42°C (10,11). However, mut-ants unable to produce histidase or urocanase are able to grow in the presence of histidine in spite of the hutI mutation.…”

Inhibition of growth by imadazol(on)e propionic acid: Evidence in vivo for coordination of histidine catabolism with the catabolism of other amino acids

“…In the first case, iopa is accumulated since it cannot be catabolized in the absence of the hutI gene product. In the second case, the hutH and U gene products are overproduced relative to the hutI gene product due to a mutation in the hutR region (Hagen et al, 1975). The toxicity of iopa has been used to select for secondary mutations alleviating the growth inhibition.…”

“…Two novel mutant classes have also been selected on the basis of iopa-resistance. One such mutant class consisted of lesions mapping in the hutC gene for the repressor protein (Hagen et al, 1975). A second class described recently consists of mutants that underproduce glutamine synthetase by virtue of lesions in glnF (Gaillardin and Magasanik, 1978).…”

Deoxyribonucleic acid-binding studies on the hut repressor and mutant forms of the hut repressor of Salmonella typhimurium

“…In addition to wild-type and repressor-negative alleles of the hutC locus, two super-repressor alleles, hutC183 and hutC161, have recently been described (3). Enzyme assays of cells bearing the two super-repressor alleles suggested that the two super-repressors, although similar in several respects (most notably, their insensitivity to inducer), apparently differ from each other and from the wild-type repressor in their affinities for the hut operators.…”

“…Media. LPC broth (5), succinate-ammonia minimal media (3,14), and histidine minimal medium (3,15) have been described; L-histidine has been added at 0.2% (wt/vol) to the succinate-ammonia minimal medium to obtain induction of the hut operons.…”