A 'natural' mutation in Saccharomyces cerevisiae strains derived from S288c affects the complex regulatory gene HAP1 (CYP1).

The HAP1 gene encodes a complex transcriptional regulator of many genes involved in electron-transfer reactions and is essential in anaerobic or heme-depleted conditions. We show here that strains derived from S288c carry a defective Ty1 element inserted in the 3' region of the HAP1 ORF. This mutant allele acts as a HAP1 null allele in terms of cytochrome c expression and CYC1 UAS1-dependent transcription, but is able to sustain limited growth in heme-depleted conditions.