Molecular cloning of chick pineal tryptophan hydroxylase and circadian oscillation of its mRNA levels.

We have previously shown that the level of [35S]methionine incorporation into tryptophan hydroxylase (TPH) shows a circadian rhythm in cultured chick pineal cells. The TPH protein oscillation persists in constant darkness, peaks in the early night and can be phase-shifted by light, in parallel to the effect of these treatments on melatonin synthesis. We have cloned and sequenced a full-length cDNA for chick pineal TPH. Levels of TPH mRNA show a robust diurnal oscillation both in vivo and in vitro. The rhythm in TPH mRNA also persists in constant darkness, suggesting that TPH mRNA synthesis and/or turnover is regulated by an endogenous circadian clock in cultured chick pineal cells. The circadian oscillation of TPH constitutes the first described circadian rhythm of a chick pineal gene at the mRNA level.