A PHOSPHORYLATED DNA-BINDING PROTEIN IS SPECIFIC FOR THE RED-LIGHT SIGNAL DURING COMPLEMENTARY CHROMATIC ADAPTATION IN CYANOBACTERIA

Complementary chromatic adaptation is a mechanism by which some cyanob acteria that are able to synthesize phycoerythrin can adapt their pigm ent (phycobiliprotein) content to the incident wavelengths of the ligh t. In Calothrix sp. PCC 7601 it concerns phycoerythrin (cpe operon), s ynthesized under green light, and phycocyanin-2 (cpc2 operon), express ed under red fight, and involves transcriptional controls. With cell-f ree extracts from Calothrix sp. PCC 7601 grown under various light reg imes, a protein designated RcaD was found by gel retardation experimen ts to specifically bind to the cpc2 promoter region and to be present only in red-light-grown cells. This protein was partially purified and its binding activity was shown to be sensitive to an alkaline phospha tase treatment. RcaD can protect two regions of the cpc2 promoter sequ ence against degradation by DNase I. Because its activity is detected only under the conditions required for cpc2 expression, we propose tha t RcaD is a positive effector of transcription.