Post-transcriptional regulation contributes to Drosophila clock gene mRNA cycling

The period (per) and timeless (tim) genes are intimately involved in the generation and maintenance of Drosophila circadian rhythms. Both genes are expressed in a circadian manner, and the two proteins (PER and TIM) participate in feedback regulation which contributes to the mRNA oscillations. Previous studies indicate that the circadian regulation is in part transcriptional. To investigate quantitative features of per and tim transcription, we analyzed the in vivo transcription rate in fly-head nuclei with a nuclear run-on assay. The results show a robust transcriptional regulation, which is similar but not identical for the two genes. In addition, per mRNA levels are regulated at a post-transcriptional level. This regulatory mode makes a major contribution to the per mRNA oscillations from a previously described per transgenic strain as well as to the mRNA oscillations of a recently identified Drosophila circadianly regulated gene (Crg-1). The data show that circadian mRNA oscillations can take place without evident transcriptional regulation.