Diurnal Variation of the Serine Dehydratase mRNA Level in Rat Liver

Serine dehydratase mRNA in the livers of rats maintained on laboratory chow (containing 22% protein) under a 12-h light (7:00-19:00)/12-h dark (19:00-7:00) cycle showed a daily oscillation that was maximal at 19:00 and minimal at 7:00 with an amplitude of more than 20-fold. The feeding of high-protein diets (60 and 91% casein) did not affect the oscillation phase. Virtually identical oscillatory phases for the mature serine dehydratase mRNA and its precursor RNAs, and the results of nuclear run-off transcription assays indicated that the oscillation was generated at the level of transcription. Similar oscillation patters were seen in adrenalectomized, starved, or diabetic rats. However, the mRNA oscillation was not precisely reflected in the oscillation of enzyme activity or enzyme protein in both the phase and the amplitude. Rats maintained on laboratory chow under constant light for 2 weeks still showed mRNA oscillation with a peak at 3:00, whereas rats exposed to constant light and fed between 7:00 and 9:00 exhibited an oscillation peak at 3:00-7:00. Under the conditions of a 12-h light/dark cycle and a 2-h restricted feeding between 7:00 and 9:00, a broad peak was seen at 23:00-3:00. These results indicate that the rhythmic expression of serine dehydratase gene is controlled by both meal-responsive and photoresponsive regulators.