IDENTIFICATION OF A NOVEL VERTEBRATE CIRCADIAN CLOCK-REGULATED GENE ENCODING THE PROTEIN NOCTURNIN

Photoreceptors of the Xenopus laevis retina are the site of a circadia n clock Ar part of a differential display screen for rhythmic gene pro ducts in this system, we have identified a photoreceptor-specific mRNA expressed in peak abundance at night, cDNA cloning revealed an open r eading frame encoding a putative 355 amino acid protein that we have n amed ''nocturnin'' (for night-factor). This protein has strong sequenc e similarity to the C-terminal domain of the yeast transcription facto r, CCR4, as well as a leucine zipper-like dimerization motif, Nocturni n mRNA levels exhibit a high amplitude circadian rhythm and nuclear ru n-on analysis indicates that it is controlled by the retinal circadian clock at the level of transcription, Our observations suggest that no cturnin may function through protein-protein interaction either as a c omponent of the circadian clock or as a downstream effector of clock f unction.