An Arabidopsis circadian clock component interacts with both CRY1 and phyB

Most organisms, from cyanobacteria to mammals, use circadian clocks to coor dinate their activities with the natural 24-h light/dark cycle. The clock p roteins of Drosophila and mammals exhibit striking homology but do not show similarity with clock proteins found so far from either cyanobacteria or N eurospora(1,2). Each of these organisms uses a transcriptionally regulated negative feedback loop in which the messenger RNA levels of the clock compo nents cycle over a 24-h period. Proteins containing PAS domains are invaria bly found in at least one component of the characterized eukaryotic clocks( 1). Here we describe ADAGIO1 (ADO1), a gene of Arabidopsis thaliana that en codes a protein containing a PAS domain. We found that a loss-of-function a do1 mutant is altered in both gene expression and cotyledon movement in cir cadian rhythmicity. Under constant white or blue light, the ado1 mutant exh ibits a longer period than that of wild-type Arabidopsis seedlings, whereas under red light cotyledon movement and stem elongation are arrhythmic. Bot h yeast two-hybrid and in vitro binding studies show that there is a physic al interaction between ADO1 and the photoreceptors CRY1 and phyB. We propos e that ADO1 is an important component of the Arabidopsis circadian system.