mCRY1 and mCRY2 are essential components of the negative limb of the circadian clock feedback loop

We determined that two mouse cryptochrome genes, mCry1 and mCry2, act in th e negative limb of the clock feedback loop. In cell lines, mPER proteins (a lone or in combination) have modest effects on their cellular location and ability to inhibit CLOCK:BMAL1-mediated transcription. This suggested crypt ochrome involvement in the negative limb of the feedback loop. Indeed, mCry 1 and mCry2 RNA levels are reduced in the central and peripheral clocks of Clock/Clock mutant mice. mCRY1 and mCRY2 are nuclear proteins that interact with each of the mPER proteins, translocate each mPER protein from cytopla sm to nucleus, and are rhythmically expressed in the suprachiasmatic circad ian clock. Luciferase reporter gene assays show that mCRY1 or mCRY2 alone a brogates CLOCK:BMAL1-E box-mediated transcription. The mPER and mCRY protei ns appear to inhibit the transcriptional complex differentially.