Haa1, a protein homologous to the copper-regulated transcription factor Ace1, is a novel transcriptional activator.

The Saccharomyces cerevisiae genome contains a predicted gene, YPR008w, homologous to the gene encoding the copper-activated transcription factor Ace1. The product of the YPR008w gene, designated Haa1, regulates the transcription of a set of yeast genes, many of which encode membrane proteins. Two main target genes of Haa1 are the multidrug resistance gene YGR138c and the YRO2 homolog to the plasma membrane Hsp30. Haa1 is localized to the nucleus. Haa1-induced expression of YGR138c and YRO2 appears to be direct. Induction of HAA1 using a GAL1/HAA1 fusion gene resulted in rapid galactose-induced expression of both HAA1 and target genes. Although Haa1 has a sequence very similar to the Cu-activated DNA binding domain of Ace1, expression of Haa1 target genes was found to be independent of the copper status of cells. Haa1 does not exhibit metalloregulation in cells incubated with a range of transition metal salts. Haa1 does not exhibit any cross-talk with Ace1. Overexpression of Haa1 does not compensate for cells lacking a functional Ace1. The lack of metalloregulation of Haa1 despite the strong sequence similarity to the copper regulatory domain of Ace1 is discussed.