Transcriptional regulation by a circadian rhythm.

Transcription of genes encoding the light-harvesting chlorophyll a/b protein (Cab) was shown previously to be regulated by light and a number of developmental factors. I show that a circadian rhythm in transcriptional activity is superimposed on these other regulatory programs. In vitro transcription measurements in isolated maize leaf nuclei demonstrated that changes in transcription are not coincident with light-dark transitions and that diurnal changes in transcription continue in continuous light and disappear in complete darkness. Light intensity influences the amplitude of transcriptional changes but has no effect on periodicity. Major diurnal changes in Cab mRNA are evident not only in developing seedling leaves, but also in mature, fully expanded leaves. Genes encoding phosphoenolpyruvate carboxylase and the cytosolic form of aldolase show no diurnal changes in transcription.