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

Genetic and physical maps of Klebsiella aerogenes genes for histidine utilization (hut)

Deletion derivatives of the hut-containing plasmid pCB101 were tested against point mutants defective in individual genes of the histidine utilization (hut) operons using a complementation/recombination assay. Location of the genes of the right operon, hutU and hutH, was confirmed by direct assay of the gene products, urocanase and histidase; location of the repressor gene was identified by measuring the ability of the plasmid-carried genes to repress the formation of histidase from a chromosomal location. The analysis of eight deletion plasmids unambiguously confirms the map order of the hut genes as hutI-G-C-U-H, and demonstrates that, in Klebsiella aerogenes, the hutU and hutH genes are transcribed from their own promoter. In addition, the genetic map of hut can be aligned with the restriction map of the hut DNA in plasmid pCB101. One of the deletion plasmids studied apparently encodes a defective histidase subunit that is trans-dominant to active histidase. Another deletion, which completely removes the left operon, hutIG, allows high level expression of the hutUH operon and thus overproduction of a toxic intermediate.

The genetics of Sleek: a possible regulatory mutation of the tabby-crinkled-downless syndrome

A new mutation,Sleek, similar in appearance to mutations at theTa, cranddlloci, has been investigated. It is inherited as an autosomal dominant and maps very close todlon chromosome 13. Allelism withdlseems probable sinceSleekinteracts withdlbut not withcr. The unusual occurrence of dominant and recessive alleles at the same locus which produce a similar mutant phenotype suggests that the locus might either code for a multimeric protein or a regulatory product.

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

In Salmonella typhimurium the genes coding for the enzymes of histidine utilization (hut) are clustered in two adjacent operons, hutMIGC and hut(P,R,Q)UH. A single repressor, the product of the C gene, regulates both operons by binding at two operator sites, one near M and one in (P,R,Q). The deoxyribonucleic acid (DNA)-binding activity of the repressor was measured using DNA's containing separate operators. The repressor had greater activity when assayed using DNA containing the operator of the (P,R,Q)UH operon than when assayed using DNA containing the operator of the MIGC operon. The binding to either operator was absent in the presence of the inducer, urocanate. The DNA-binding activities were also determined for two super-repressors. The super-repressors had altered DNA-binding properties, although the self-regulated nature of the repressors complicated the analysis of the results. A purfication procedure for the wild-type repressor is presented. The purified repressor was somewhat unstable, and additional experiments using it were not performed.

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

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 imidazole propionate. 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.