Genetics of sulfate transport by Salmonella typhimurium

Sixty-four mutants were isolated from the LT-2 wild-type strain of Salmonella typhimurium by selecting for chromate resistance. The majority of lesions were shown to lie in the cysA gene. (i) The mutants cannot take up sulfate, a finding which verifies the role of cysA in sulfate transport. In addition, 52 sulfate-transport mutants isolated without chromate selection were defective in the cysA gene. (ii) Most had less than 25% of the binding activity of the wild-type strain. (iii) Most had normal sulfite reductase (H(2)S-nicotinamide adenine dinucleotide phosphate oxidoreductase, EC 1.8.1.2) activity. (iv) Their sulfate-binding protein (binder) appears electrophoretically and immunologically normal. (v) Amber cysA mutants also make apparently normal binder in small amounts. (vi) All classical cysA mutants tested, including two with long deletions, had normal binding activity. From these observations, it is suggested that the cysA gene does not code for the binder. But many mutations in this gene reduce the binding activity in some unknown way. Other mutants, identified as cysB mutants, had neither binding nor uptake activities and their sulfite reductase activities were similarly reduced, thus confirming the regulatory role of the cysB gene. When binder was detectable, it had wild-type properties. No mutations in the binder gene were found among more than 100 mutants examined. Thus, sulfate binding has not been established as a part of sulfate transport. However, the production of binder is intimately connected with cysA, the established sulfate transport gene, and is regulated by the same mechanism that regulates both transport and the rest of the cysteine biosynthetic pathway.