MOLECULAR INTERACTION BETWEEN COP1 AND HY5 DEFINES A REGULATORY SWITCH FOR LIGHT CONTROL OF ARABIDOPSIS DEVELOPMENT

Arabidopsis COP1 acts as a light-inactivable repressor of photomorphog enic development, but its molecular mode of action remains unclear. He re, we show that COP1 negatively regulates HY5, a bZIP protein and a p ositive regulator of photomorphogenic development. Both in vitro and i n vivo assays indicate that COP1 interacts directly and specifically w ith HY5. The hyper-photomorphogenic phenotype caused by the overexpres sion of a mutant HY5, which lacks the COP1-interactive domain, support s the regulatory role of HY5-COP1 interaction. Further, HY5 is capable of directly interacting with the CHS1 minimal promoter and is essenti al for its light activation. We propose that the direct interaction wi th and regulation of transcription factors by COP1 may represent the m olecular mechanism for its control of gene expression and photomorphog enic development.