ENZYMATIC DEFECTS IN SULFATE REDUCING SYSTEM OF SULFITE-LESS YEAST MUTANTS

Among a number of methionine-dependent mutants of Saccharomyces cerevisiae, which were isolated from an ultraviolet-irradiated wild strain of the yeast, we found five sulfite-dependent mutants which were of interest as materials for the study of pathway of sulfate reduction to sulfite. By measuring the enzymic activities of their cell extracts, two of them were found to be deficient in the ATP-sulfurylase system, one in the APS-kinase system, and the remaining two in the PAPS reducing system. Sulfite formation could be caused by adequate combination of these systems, but not by combination of the extracts which were lacking in the same enzymic system. The suspected subunits of PAPS reducing system could not be differentiated by the mutants at hand. The reduction of sulfate to sulfite is considered to be the first obligatory step in the synthetic pathway leading from sulfate to organic sulfur compounds in various microorganisms (1-7). The reactions known for the sulfate reduction in bakers ' yeast (8, 9) are: SO'~+ATPc APS+PP (I) ATP-sulfurylas