The N-terminal fragment of Arabidopsis photomorphogenic repressor COP1 maintains partial function and acts in a concentration-dependent manner

Arabidopsis seedlings exhibit distinct developmental patterns according to their light environment: photomorphogenesis in the light and etiolation or skotomorphogenesis in darkness. COP1 acts within the nucleus to repress pho tomorphogenesis in darkness, while light depletes COP1 from nucleus and abr ogates this repression. COP1 contains three structural modules: a RING fing er followed by a coiled-coil domain, and a WD40 repeat domain at the C-term inus. By introducing various domain deletion mutants of COP1 into cop1 null mutant backgrounds, we show that all three domains are essential for the f unction of COP1 in vivo. Interestingly, a fragment containing the N-termina l 282 amino acids of COP1 (N282) with both the RING finger and coiled-coil modules is sufficient to rescue the lethality of the cop1 null mutations at low expression level. However, high expression levels of the N282 fragment result in a phenocopy of the cop1 null mutation. The sensitivity of the se edling to levels of N282 could reflect the importance of the abundance of C OP1 for the appropriate regulation of photomorphogenic development.