Zebrafish Clock rhythmic expression reveals independent peripheral circadian oscillators

The only vertebrate clock gene identified by mutagenesis is mouse Clock, wh ich encodes a bHLH-PAS transcription factor. We have cloned Clock in zebraf ish and show that, in contrast to its mouse homologue, it is expressed with a pronounced circadian rhythm in the brain and in two defined pacemaker st ructures, the eye and the pineal gland. Clock oscillation was also found in other tissues, including kidney and heart. In these tissues, expression of Clock continues to oscillate in vitro. This demonstrates that self-sustain ing circadian oscillators exist in several vertebrate organs, as was previo usly reported for invertebrates.