One of our goals is to define the biochemical machinery
that underlies the mysterious yet ubiquitous process of
circadian rhythmicity. Our entrèe into the process
was the period gene (per) of Drosophila malanogaster.
The per protein (PER) mutants have profound effects on
circadian rhythms of locomotor activity and on circadian
rhythms of eclosion (emergence of adults from the pupal
case). More than a decade ago, we discovered that per
MRNA as well as PER undergoes circadian fluctuations in
level during the circadian cycle. These observations and
others indicate that there is a negative-feedback loop,
in which PER and its partner protein TIM inhibit the transcription
of their own mRNAs. Negative feedback at the transcriptional
level is now accepted as a central feature of circadian
timekeeping in plants, cyanobacteria, Neurospora,
and mammals. This indicates that the principles-if not
the components-of the Drosophila clock are widely
conserved.
Since 1998, the number of known Drosophila central
clock genes has increased to approximately eight; the
precise number depends a bit on the definition of "clock
gene," which is interpreted differently in different quarters.
Two of these genes are kinases, and there is increasing
evidence for the importance of post- transcriptional regulation
in circadian timekeeping. My laboratory is studying the
role of the circadian transcriptional factor genes, Clock
(Clk) and cycle (cyc). The two protein products, CLK and
CYC, comprise the key heterodimeric transcription 23 factor
that drives per and tim transcription. It also drives
the transcription of a number of downstream clock functions.
We have been actively pursuing the identification as well
as function of circadian genes, using microarray and other
methods. Our intention is to subdivide new circadian genes
into those that are direct targets of the CLK-CYC complex
and those that are indirect targets. Some may even turn
out to be central clock genes, and this approach should
complement our continuing efforts to identify central
clock genes with more traditional genetic strategies.
The mammalian CLK ortholog was first identified as a mouse
circadian clock mutant in Joseph Takahashi's laboratory
(HHMI, Northwestern University). The mouse and fly mutants
have strikingly similar phenotypes and etiologies, making
it likely that target genes as well as mechanisms will
be shared between flies and mammals.
