O-acetyl homoserine-O-acetyl serine sulfhydrylase; involved in methionine and cysteine biosynthesis; long considered essential for inorganic sulfur assimilation, but inefficient homocysteine synthase Hsu1p enables met17 null cells to assimilate enough inorganic sulfur for survival and proliferation; growth failure of met17 null can be explained by toxic accumulation of H2S gas due to a metabolic bottleneck