Molecular genetics of met17 and met25 mutants of Saccharomyces cerevisiae: Intragenic complementation between mutations of a single structural gene

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Abstract
We cloned the MET17 gene of Saccharomyces cerevisiae by functional complementation after transformation of a yeast met17 mutant. Restriction mapping and nucleotide sequencing of the MET17 clones revealed that these were from the same genomic region as clones isolated previously and shown to contain the MET25 gene encoding the enzyme O-acetylhomoserine, O-acetylserine sulphydrylase (OAH-OAS sulphydrylase). Transformation studies with MET25 clones showed that the MET17 and MET25 functions were both endoced in a single transcription unit. We conclude that met17 and met25 are both mutations in the structural gene for the OAH-OAS sulphydrylase subunit and that each affects a different fuctional domain of the enzyme allowing subunit complementation in the met17xmet25 diploid. Enzyme assays indicated that the diploid, although not requiring methionine, had a low OAH-OAS sulphydrylase activity (10% of wild type). This is consistent with MET17 and MET25 being the same gene. We found that both met17 and met25 mutants were devoid of 3′ phospho-adenosine 5′ phospho-sulphite (PAPS) reductase activity and that this activity was fully restored in the met17xmet25 diploid. The possible interactions between OAH-OAS sulphydrylase and PAPS reductase are discussed.