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 Cpx1 genes (encoding cytochrome c(6) and coprogen oxidase) in response to copper deficiency. Mutational analysis of promoter regions of the Cyc6 and Cpx1 genes revealed a four nucleotide sequence, GTAC, which was absolutely essential for copper responsiveness. The Cyc6 promoter contains two copper response elements, each with a functionally important GTAC sequence, whereas the Cpx1 promoter contains only one. This may contribute to the stronger and more tightly regulated expression of the Cyc6 gene. Mutation or deletion of sequences flanking the GTACs implicates additional nucleotides contributing to copper-responsive expression, but none are absolutely essential. Metal ion selectivity of Cpx1 expression is identical to that described previously for Cyc6 and is restricted to the copper deficiency-induced Cpx1 transcript. The Cyc6 and Cpx1 genes are also induced by oxygen deficiency. Reporter gene constructs indicate that the induction occurs at the level of transcription and requires the same GTAC sequence that is critical for copper responsiveness. We suggest that components of the copper-responsive signal transduction pathway are used for some of the changes in gene expression in hypoxic cells.