Regulatory mutants and control of cysteine biosynthetic enzymes in Salmonella typhimurium.

The cysB region in Salmonella typhimurium regulates in a positive manner the noncontiguous structural genes for the enzymes responsible for sulfate reduction in cysteine biosynthesis. We treated three cysB mutants with chemical mutagens and selected 81 secondary mutants in which the inability to utilize sulfate was suppressed. Growth experiments on the suppressed mutants showed that the original loss of sulfate utilization had been corrected to varying degrees and that portions of the pathway had been established in abnormal relationship to one another. Sixty of the suppressed mutations were mapped via transductional analysis, and each was very closely linked to the original cysB mutation. We demonstrated that the cysB product functions in the regulation of the cysteine biosynthetic enzymes during both logarithmic growth and stationary phase. Mutation can alter the regulatory response of one enzyme in either an upward or downward direction while the regulation of other enzymes in the pathway remains unchanged. These data are consistent with the idea of a multivalent or multisite regulator molecule.