Mutation of negative regulatory gene encoding an FBXO3 protein results in normoxic expression of genes in .

Oxygen is known to prevent hydrogen production in Chlamydomonas, both by inhibiting the hydrogenase enzyme and by preventing the accumulation of HYDA-encoding transcripts. We developed a screen for mutants showing constitutive accumulation of transcripts in the presence of oxygen. A reporter gene required for ciliary motility, placed under the control of the promoter, conferred motility only in hypoxic conditions. By selecting for mutants able to swim even in the presence of oxygen we obtained strains that express the reporter gene constitutively. One mutant identified a gene encoding an F-box only protein 3 (FBXO3), known to participate in ubiquitylation and proteasomal degradation pathways in other eukaryotes. Transcriptome profiles revealed that the mutation, termed , leads to constitutive expression of and other genes regulated by hypoxia, and of many genes known to be targets of CRR1, a transcription factor in the nutritional copper signaling pathway. was required for the constitutive expression of the reporter gene in mutants. The CRR1 protein, which is normally degraded in Cu-supplemented cells, was stabilized in cells, supporting the conclusion that acts to facilitate the degradation of CRR1. Our results reveal a novel negative regulator in the pathway and possibly other pathways leading to complex metabolic changes associated with response to hypoxia.