2 PERIOD HOMOLOGS - CIRCADIAN EXPRESSION AND PHOTIC REGULATION IN THESUPRACHIASMATIC NUCLEI

We have characterized a mammalian homolog of the Drosophila period gen e and designated it Per?. The PERS protein shows >40% amino acid ident ity to the protein of another mammalian per homolog (designated Per1) that was recently cloned and characterized. Both PER1 and PER2 protein s share several regions of homology with the Drosophila PER protein, i ncluding the protein dimerization PAS domain. Phylogenetic analysis su pports the existence of a family of mammalian per genes. In the mouse, Per1 and Per2 RNA levels exhibit circadian rhythms in the SCN and eye s, sites of circadian clocks. Both Per1 and Per2 RNAs in the SCN are i ncreased by light exposure during subjective night but not during subj ective day. The results advance our knowledge of candidate clock eleme nts in mammals.