Coordinate copper- and oxygen-responsive Cyc6 and Cpx1 expression in Chlamydomonas is mediated by the same element

Chlamydomonas reinhardtii activates the transcription of the Cyc6 and the C px1 genes (encoding cytochrome c(6) and coprogen oxidase) in response to co pper deficiency. Mutational analysis of promoter regions of the Cyc6 and Cp x1 genes revealed a four nucleotide sequence, GTAC, which was absolutely es sential for copper responsiveness. The Cyc6 promoter contains two copper re sponse elements, each with a functionally important GTAC sequence, whereas the Cpx1 promoter contains only one. This may contribute to the stronger an d more tightly regulated expression of the Cyc6 gene. Mutation or deletion of sequences flanking the GTACs implicates additional nucleotides contribut ing to copper-responsive expression, but none are absolutely essential. Met al ion selectivity of Cpx1 expression is identical to that described previo usly for Cyc6 and is restricted to the copper deficiency-induced Cpx1 trans cript, The Cyc6 and Cpx1 genes are also induced by oxygen deficiency. Repor ter gene constructs indicate that the induction occurs at the level of tran scription and requires the same GTAC sequence that is critical for copper r esponsiveness. We suggest that components of the copper-responsive signal t ransduction pathway are used for some of the changes in gene expression in hypoxic cells.