Figure 1.

Although we know much about the biosynthesis of melatonin by the avian pineal gland, we know nothing of the clock that generates its rhythm. (A) The biosynthetic mechanism is regulated transcriptionally by a circadian clock that presumably resides in the nucleus. Rhythms of TrH, AANAT, and HIOMT activity are driven by rhythms in their mRNA levels. (B) A generalized schematic of the interactions of the putative components of the mammalian molecular clock. Positive elements BMAL1 and CLK drive transcription of the negative elements per1, per2, per3, cry1, and cry2. They then are translated, dimerize and, on transport back into the nucleus, disrupt positive regulation of clock gene transcription. Does a similar mechanism regulate avian melatonin rhythms?