Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 213

PER

PER

PER
TIM

CYC CLK

TIM

per/tim

Nucleus

TIM

Cytoplasm

per/tim mRNAs PER/TIM proteins

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Time of day

Current Biology

Figure 4.8 Overview of the Drosophila molecular circadian oscillator. Expression of the per
and tim genes is promoted by the heterodimeric CLK-CYC transcription factors and reaches
a peak late in the day. Translation of per and tim RNAs leads to the gradual accumulation and
dimerization of PER and TIM proteins within the cytoplasm. The protein levels peak at night,
during which time they separately enter the nucleus to inhibit further CLK-CYC transcriptional
activity as shown. Abbreviations : CLK, clock; CYC, cycle; per, period; tim, timeless.
Source : From Nitabach and Taghert (2008).

in Drosophila , including photoreceptor neurons, which are exposed to daylight,
express clk and cyc ? As a first impression, it seems paradoxal that clk and cyc genes
are expressed by clock neurons not directly exposed to the light, yet not expressed in
cells and neurons that are exposed. As it was explained earlier in this chapter, genes
do not understand nature's language; they have no hunch on what dawn and nightfall
presage for the fly's physiology, survival, and reproduction. To make these environ-
mental messages intelligible to genes, the messages must be translated into a lan-
guage the genes understand and the polyglot translator is in the brain.
The hierarchical character of the circadian mechanisms is clearer in mammals,
where the central neural clock controls and resets the peripheral clocks. The timing