Pleiotrophy in a cysteine-requiring mutant of Samonella typhimurium resulting from altered protein-protein interaction.

The bifunctional protein complex, cysteine synthetase, in Salmonella tyPhimurium is composed of the enzymes serine transacetylase and 0-acetylserine sulfhydrylase. A point mutation (BBl) in the structural gene for serine transacetylase results in diminished catalytic activity of both components of the complex. Enzyme inactivation studies using either specific 0-acetylserine sulfhydrylase antiserum or heat show that the isolated 0-acetylserine sulfhydrylase components of the mutant are indistinguishable from wild type O-acetylserine sulfhydrylase. When 0-acetylserine sulfhydrylase from either wild type or mutant source is combined with serine transacetylase from a common source, their behavior is identical as measured by decrement in enzymic activity, immunological inactivation, and heat inactivation. Thus, when hybrid cysteine synthetases, composed of one component from the wild type and one from the mutant, are investigated, the 0-acetylserine sulfhydrylase components of such hybrid complexes behave like those found in the native cysteine synthetase from which the serine transacetylase component was derived. These studies establish that the diminished catalytic activity of the sulfhydrylase of the mutant results from the association of catalytically normal O-acetylserine sulfhydrylase with mutant serine transacetylase. The basis of the pleiotropic phenotype of mutant BBl is, therefore, an alteration in protein-protein interaction resulting from a structural gene mutation.