Repression of photomorphogenesis in Arabidopsis thaliana requires activity of 
the COP9 signalosome (CSN), CDD, and COP1 complexes, but how these three 
complexes work in concert to accomplish this important developmental switch has 
remained unknown. Here, we demonstrate that Arabidopsis CULLIN4 (CUL4) 
associates with the CDD complex and a common catalytic subunit to form an active 
E3 ubiquitin ligase both in vivo and in vitro. The partial loss of function of 
CUL4 resulted in a constitutive photomorphogenic phenotype with respect to 
morphogenesis and light-regulated gene expression. Furthermore, CUL4 exhibits a 
synergistic genetic interaction with COP10 and DET1. Therefore, this CUL4-based 
E3 ligase is essential for the repression of photomorphogenesis. This CUL4-based 
E3 ligase appears to associate physically with COP1 E3 ligase and positively 
regulates the COP1-dependent degradation of photomorphogenesis-promoting 
transcription factors, whereas the CSN controls the biochemical modification of 
CUL4 essential for E3 activity. Thus, this study suggests a biochemical activity 
connection between CSN and CDD complexes in their cooperation with COP1 in 
orchestrating the repression of photomorphogenesis.