The principal clock gene in Neurospora is frequency (frq).
The levels of both frq mRNA and FRQ protein cycle, with the peak
levels occurring about 10 to 12 hours after midnight (Dunlap 1999). The
levels of FRQ peak about 4 hours after the mRNA, with the delay being due to
post-transcriptional delays (Garceau et al. 1997). FRQ is then translocated
in to the nucleus where it acts as a negative repressor on frq. This
causes a rapid downregulation of frq so that by late day frq is not
transcribed at all. This repression is maintained through the night during
which time FRQ is increasingly phosphorylated. This acts as a degradation
signal so that as the night progresses the levels of FRQ begin to drop,
relieving the repression of frq (Green 1998).
FRQ contains certain sequence similarities to transcription factors,
suggesting that this is its function.
Expression of FRQ requires the presence of the proteins White
Collar-1 (WC-1) and White Collar-2 (WC-2). These contain domains called PAS
domains (click here for a description
of PAS domains). (Harmer et al 2001). WC-1 and WC-2 are involved in the perception of blue light, the only form
of light Neurospora responds to, and may also be involved in gene
activation (Talora et al 1999). WC-1 and WC-2 dimerise in vitro at
the PAS domain and act as transcription factors and stimulate the
transcription of frq. As such it is assumed that they bind to the frq
promoter, although this has never been proven. It is known that the FRQ protein binds to
WC-2. This suggests that FRQ inhibits its own transcription
by blocking the action of WC-1 and WC-2 (Merrow 2001).
The
Neurospora clock. Proteins with positive actions are in yellow, negative
components are in blue. Protein-protein interactions are indicated with
dotted black arrows. Green arrows indicate a positive effect on some process
or promoter, and red lines indicate an inhibitory action. The question
marks indicate that the white-collar complex has not been shown to
directly bind the promoters of frq or other clock-controlled genes.
(From
Harmer 2001)
CLOCK
GENES IN