Analysis of merodiploids of the cysB region in Salmonella typhimurium.

Summary We have studied the regulation of two cysteine biosynthetic enzymes in S. typhimurium merodiploid strains which are heterozygous at the cysB regulatory locus. This gene codes for an element of positive control which is necessary for the expression of the enzymes of the biosynthetic pathway. Under conditions of sulfur deprivation levels of sulfite reductase (coded for by cysI, cysJ and cysG) and of O-acetylserine sulfhydrylase (coded for by cysK) are derepressed in cysB ⁺ haploid strains, but not in cysB ^- haploid strains. Growth on a rich sulfur source such as l-cystine results in low levels of both enzyme activities in cysB ⁺ and cysB ^- haploid strains but not in cysB ^c haploid strains, where enzyme expression is constitutive, i.e. substantially greater than in a cysB ⁺ strain grown on l-cystine, regardless of the nutrients used for growth.We find that cysB ^-/F cysB ⁺ merodiploid strains can be derepressed for sulfite reductase and O-acetylserine sulfhydrylase by growth on a poor sulfur source, and therefore cysB ⁺ is dominant to cysB ^-. Enzyme levels are also derepressed in l-cystine-grown cysB ^c/F cysB ⁺ strains indicating that cysB ^c is dominant to cysB ⁺. The cysB484 allele is known to be cysB ^- in regard to the regulation of sulfite reductase activity, but cysB ^c with respect to O-acetylserine sulfhydrylase. In a cysB484/F cysB ⁺ strain the cysB ^- character of cysB484 is recessive to cysB ⁺, while cysB ^c is dominant to cysB ⁺.Merodiploids of the type cysB ^-/F cysB ⁺, bearing chromosomal point mutations are derepressed by sulfur deprivation to levels which are either less than, equal to, or greater than those of wild type. These results can be explained by assuming a multimeric structure for the cysB protein and the formation in merodiploids of cysB ^-/cysB ⁺ hybrid molecules with altered capacities for gene activation. The dominance of cysB ^c over cysB ⁺ indicates that in contrast to the araC regulatory protein, which acts as both a gene activator and repressor, the cysB protein serves only as an element of positive control.