Diurnal change of light-dependent uridine incorporation into RNA in a long-day duckweed, Lemna gibba G3

Light-dependent incorporation during subjective day and nightof radioactive uridine into RNA of a long-day duckweed, Lemmagibba G3, was examined. When the dark treatment was startedfrom the subjective night phase, the activity of uridine incorporationdropped approximately by half only after the very subjectivenight phase had passed or with the commencement of the subsequentsubjective day phase. However, when the dark treatment was startedfrom the subjective day phase, the incorporating activity promptlybegan to decrease and the inhibition increased with the lengthof the dark period until a final steady level (also at ca. 50%of the initial level) was reached after 24 hr of darkness. Thesetwo phases of different light sensitivities recurred daily undercontrol of the physiological clock and the rhythm was resetby a light-on signal. The lowered incorporating activity dueto the darkened day phase was recovered completely by a 12-hror even 15-min white light period perturbing the succeedingnight phase. That the incorporation of uridine in every RNAspecies, especially in chloroplast ribosomal RNA, was loweredby dark treatment of the day and night phases, was disclosedby MAK column chromatography and acrylamide gel electrophoresis. (Received August 21, 1974; )