Genetic dissection of nutritional copper signaling in Chlamydomonas distinguishes regulatory and target genes

Abstract

A genetic screen for Chlamydomonas reinhardtii mutants with copper-dependent growth or nonphotosynthetic phenotypes revealed three loci, COPPER RESPONSE REGULATOR 1 (CRRl), COPPER RESPONSE DEFECT 1 (CRD1), and COPPER RESPONSE DEFECT 2 (CRD2), distinguished as regulatory or target genes on the basis of phenotype. CRRl was shown previously to be required for transcriptional activation of target genes like CYC6, CPXl, and CPD1, encoding, respectively, cytochrome C6 (which is a heme-containing substitute for copper-containing plastocyanin), coproporphyrinogen III oxidase, and Mg-protoporphyrin IX monomethylester cyclase. We show here that CRR1 is required also for normal accumulation of copper proteins like plastocyanin and ferroxidase in copper-replete medium and for apoplastocyanin degradation in copper-deficient medium, indicating that a single pathway controls nutritional copper homeostasis at multiple levels. CRRl is linked to the SUPPRESSOR OF PCY1-AC208 13 (SOP13) locus, which corresponds to a gain-of-function mutation resulting in copper-independent expression of CYC6. CRRl is required also for hypoxic growth, pointing to a physiologically meaningful regulatory connection between copper deficiency and hypoxia. The growth phenotype of Ctrl strains results primarily from secondary iron deficiency owing to reduced ferroxidase abundance, suggesting a role for CRRl in copper distribution to a multicopper ferroxidase involved in iron assimilation. Mutations at the CRD2 locus also result in copper-conditional iron deficiency, which is consistent with a function for CRD2 in a pathway for copper delivery to the ferroxidase. Taken together, the observations argue for a specialized copper-deficiency adaptation for iron uptake in Chlamydomonas.