A MYB-type transcription factor, MYB2, represses light-harvesting protein genes in Cyanidioschyzon merolae

While searching for transcriptional regulators that respond to changes in light regimes, we identified a MYB domain-containing protein, MYB2, that accumulates under dark and other conditions in the unicellular red alga Cyanidioschyzon merolae. The isolation and analysis of a MYB2 mutant revealed that MYB2 represses the expression of the nuclear-encoded chloroplast RNA polymerase sigma factor gene SIG2, which results in the repression of the chloroplast-encoded phycobilisome genes that are under its control. Since nuclear-encoded phycobilisome and other light-harvesting protein genes are also repressed by MYB2, we conclude that MYB2 has a role in repressing the expression of light-harvesting genes. The MYB2 mutant is sensitive to a prolonged dark incubation, indicating the importance of MYB2 for cell viability in the dark.