SCREEN FOR GENES INDUCED IN THE SUPRACHIASMATIC NUCLEUS BY LIGHT

The mechanism by which mammalian circadian clocks are entrained to lig ht-dark cycles is unknown. The clock that drives behavioral rhythms is located in the suprachiasmatic nucleus (SCN) of the brain, and entrai nment is thought to require induction of genes in the SCN by light. A complementary DNA subtraction method based on genomic representational difference analysis was developed to identify such genes without maki ng assumptions about their nature. Four clones corresponded to genes i nduced specifically in the SCN by light, all of which showed gating of induction by the circadian clock. Among these genes are c-fos and nur 77, two of the five early-response genes known to be induced in the SC N by light, and egr-3, a zinc finger transcription factor not previous ly identified in the SCN. In contrast to known examples, egr-3 inducti on by light is restricted to the ventral SCN, a structure implicated i n entrainment.