The isolation and characterization of missense mutants in the general repressor protein Ssn6 of Saccharomyces cerevisiae.

Ssn6, a TPR repeat-containing protein, associates with the Tup1 protein to form a general transcriptional repression complex in Saccharomyces cerevisiae. As part of a genetic analysis of this complex, we targeted mutations to the TPR repeat-coding region of the SSN6 gene, and applied selection for constitutive expression of the hypoxic gene ANB1. All but one of the resulting mutants failed to express full-length Ssn6 protein, indicating that they harbored deletion, frameshift, or nonsense mutations. The one missense mutation encoded a protein with three amino acid substitutions, and the combination was required for the mutant phenotype. One mutation, a proline substitution for a serine at codon 51, was used in a second round of mutagenesis in which six further multiple-substitution alleles were obtained. These were separated into their component mutations, and again, all but one of the single substitutions displayed the wild-type phenotype. The single and multiple mutants were characterized in terms of their effects on the repression of the glucose-repressible SUC2 gene and the a mating-type gene STE2. The mutant Ssn6 proteins were also tested for their ability to associate with Tup1. The S5 P mutation, despite its lack of a mutant phenotype, had lost the ability to fully associate with Tupl in vitro. In general, those single substitutions that fell within the first two TPR repeats impaired Tup1-associating activity, while the two that fell in TPR repeats five and eight retained this activity. Overexpression of TUP1 partially suppressed the mutant phenotype in only some of the multiple mutants. The results are discussed in terms of the current models of Ssn6 function and the structure of TPR repeats.