The role of cysteine residues in the homeodomain protein Matα2 in mating-type control of Saccharomyces cerevisiae
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The Matα2 homeodomain protein plays a pivotal role in the control of cell type in Saccharomyces cerevisiae. The homeodomain in the C-terminal region of Matα2 functions as a DNA-binding domain and the N-terminal region, containing two cysteine residues at positions 33 and 34, is thought to be involved in formation of Matα2 homodimers via disulfide bonds. matα2 mutants, isolated in a previous study, in which haploid-specific genes cannot be repressed by the Mata1-Matα2 heterodimer but a-specific genes can be repressed by the Matα2 homodimer, were found to produce mutant Matα2 with a substitution of tyrosine or phenylalanine for Cys33. To clarify the role of Cys33 and Cys34 in the Matα2 protein, we generated several matα2 mutants by site-directed mutagenesis which had serine residues in place of these Cys residues. Transforming MATa cells with plasmids carrying these matα2 (MATα1+) mutations rendered transformants unable to mate. Northern blot analysis revealed that transcription of the a-specific gene STE2 and the haploid-specific locus RME1 in these transformants is repressed to the same level as in wild-type MATa/MATα cells. We concluded that neither Cys33 nor Cys34 is required for repression of a-specific genes by the Matα2 homodimer or of haploid-specific genes by the Mata1-Matα2 heterodimer, and therefore suggest that Matα2 homodimer formation in vivo is not mediated by disulfide linkage.
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