A MYB-RELATED TRANSCRIPTION FACTOR IS INVOLVED IN THE PHYTOCHROME REGULATION OF AN ARABIDOPSIS LHCB GENE

We have isolated the gene for a protein designated CCA1. This protein can bind to a region of the promoter of an Arabidopsis light-harvestin g chlorophyll a/b protein gene, Lhcb13, which is necessary for its re gulation by phytochrome. The CCA1 protein interacted with two imperfec t repeats in the Lhcb13 promoter, AA(A)/(C)AATCT, a sequence that is conserved in Lhcb genes. A region near the N terminus of CCA1, which h as some homology to the repeated sequence found in the DNA binding dom ain of Myb proteins, is required for binding to the Lhcb13 promoter. Lines of transgenic Arabidopsis plants expressing antisense RNA for CC A1 showed reduced phytochrome induction of the endogenous Lhcbl3 gene , whereas expression of another phytochrome-regulated gene, rbcS-1A, w hich encodes the small subunit of ribulose-1,5-bisphosphate carboxylas e/oxygenase, was not affected. Thus, the CCA1 protein acts as a specif ic activator of Lhcb13 transcription in response to brief red illumin ation, The expression of CCA1 RNA was itself transiently increased whe n etiolated seedlings were transferred to light. We conclude that the CCA1 protein is a key element in the functioning of the phytochrome si gnal transduction pathway leading to increased transcription of this L hcb gene in Arabidopsis.