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Transcription of HEM13 in Saccharomyces cerevisiae is repressed by heme and oxygen. We have isolated two mutants in which expression of HEM13 is aberrant. The mutant alleles in these strains represent two different alleles of HAP1. HAP1 encodes an activator protein whose DNA binding activity is stimulated by heme, and is required for the transcription of CYC1, ROX1 and a number of other heme-dependent genes. One of our mutant alleles confers a phenotype much like that of the hap1::LEU2 allele. Expression of HEM13 in a strain with this mutation is elevated under repressing conditions and not fully inducible in the absence of heme. The other mutant allele of HAP1 we uncovered confers a novel phenotype. A strain containing this allele exhibits heme-independent expression of CYC1 and ROX1 and uninducible expression of HEM13 and ANB1. The mutation associated with this novel allele of HAP1 was localized to a glycine to aspartate change in amino acid 235 of HAP1, between the DNA binding and heme responsive domains. DNA binding assays demonstrated that the protein made from this HAP1 allele retains the ability to bind DNA, but that unlike wild-type HAP1 protein, this binding is not stimulated by heme.

A novel allele of HAP1 causes uninducible expression of HEM13 in Saccharomyces cerevisiae.