Stable isotope fractionation by Clostridium pasteurianum. 2. Regulation of sulfite reductases by sulfur amino acids and their influence on sulfur isotope fractionation during SO32− and SO42− reduction

Abstract

In addition to an assimilatory sulfite reductase, studies of cultures of Clostridium pasteurianum supplemented with methionine, cysteine, and 35SO42− provide evidence for another reductase which is induced by SO32−. This inducible reductase appears to be dissimilatory because of the copious sulfide production arising when the cells are grown on SO32−. Cysteine can repress the assimilatory sulfite reductase but does not affect the inducible reductase. During late logarithmic growth on 1 mM SO42− + 10 mM cysteine, derepression of the inducible reductase occurred along with increased sulfide production. The presence of 1 mM cysteine and (or) 1 mM methionine does not affect the inverse sulfur isotope effect for evolved H2S. However, 5 and 10 mM cysteine reduce the maximum δ34S value for released H2S from +40 to +10‰. A small conversion of cysteine to H2S by C. pasteurianum occurs, but only in the stationary phase.