Circadian transcription profile of phosphoenolpyruvate carboxylase ( Pepc ). Quantification of (A) Pepc hnRNA and (B) Pepc mRNA during 56 hours of constant illumination. After a normal 16 h light period, illumination was extended for an additional 40 h without any dark period or temperature shift. Dark periods during previous growth are symbolized by the light grey bar under each chart. Numbers are values relative to transcription levels in leaves harvested 4 hours after illumination. Data points are based on four independent experiments. Vertical lines indicate standard errors.