Multiple Mutations in CysA 14 MUTANTS OF Bacillus Subtilis

Isolates of Bacillus subtilis that had been presumed to carry the cysA14 lesion have been studied. Our data indicate that these strains contain four mutations, all of which are linked by transformation and lie in the region of the ribosomal markers. The requirement for cysteine results from a defective serine transacetylase that is coded for by the cysA locus. Therefore, these mutants grow only in the presence of cysteine but not with sulfate, sulfite, or sulfide as the sole source of sulfur. A second genetic lesion (css) can be recognized by an increased sensitivity to the amino acid L-cysteine. The inhibited enzyme(s) has not been determined but inhibition is overcome by a mixture of eight amino acids. The third mutation (hts) results in the overproduction and excretion of hydrogen sulfide. This compound appears to be produced from cysteine by the enzyme cysteine desulfhydrylase and not by an increased activity of the sulfate-reductive pathway. This locus presumably codes for a regulatory element involved in the control of cysteine desulfhydrylase. The fourth mutation (cym) is not well characterized biochemically but results in a requirement for cysteine or methionine. The following order of these mutations has been established by transformation studies: hts, cysA, css, cym. The generally poor growth of these mutants in minimal-salts glucose media supplemented with cysteine can now be explained by these observations. The cysA14 mutants not only require an amino acid that is itself inhibitory to growth but they also overproduce the highly toxic compound hydrogen sulfide.