The overproducing CYP1 and the underproducing hap1 mutations are alleles of the same gene which regulates in trans the expression of the structural genes encoding iso-cytochromes c.

The CYP1 gene has previously been identified as coding for a positive trans active factor that activates the expression of CYC1 and CYP3, which are the structural genes for isol1- and iso2-cytochrome c. Two phenotypically distinct classes of CYP1 mutations can be obtained indicating that CYC1 and CYP3 are differentially regulated by the product of CYP1. The HAP1 gene codes for a product which has previously been proved to be necessary for the expression of the heme dependent CYC1-UAS1 cis regulatory sequence. In this article, we show by complementation and recombination that CYP1 and HAP1 are the same gene, moreover we identify hap1-1 as an iso2-cytochrome c underproducer mutation of the CYP1 gene.