Further studies on the diurnal rhythm of uridine incorporation into RNA in the long-day duckweed, Lemna gibba G3

Using the long-day duckweed Lemna gibba G3, the changes in theactivities of RNA synthesis in isolated nuclei and chloroplastsand of the reaction prerequisite for the incorporation of exogenousuridine into RNA were examined. When the duckweed was exposedto either a light-dark cycle or to continuous light, the activityof RNA synthesis in the nuclear and chloroplast fractions changeddiurnally and reached its highest levels during the night phase.The changes coincided with uridine incorporation into RNA invivo. However, the amount of radioactive uridine taken up intothe acid-soluble fraction remained unchanged during the wholeday. The proportion of radioactivity incorporated into phosphorylateduridine compounds as well as UTP+UDP to the radioactivity inthis fraction remained constant. Thus, the diurnal rhythm ofuridine incorporation into RNA was related to the diurnal rhythmof RNA synthesis in isolated nuclei and chloroplasts. The loweractivity of uridine incorporation into RNA under continuousdarkness may be determined by the activity of RNA synthesisin nuclei and chloroplasts as well as the uptake rate of uridineinto the duckweed cells, not by the activity of its phosphorylation. (Received August 30, 1977; )