Circadian rhythm of rna synthesis in rat liver: effect of protein synthesis-inhibiting antibiotics at different times of day

1. 1. The content of free nucleotides and nucleosides, determined by u.v. measurement, did not change significantly during 24 hours. 2. 2. The antibiotics cycloheximide, pactamycin, puromycin and fusidic acid inhibited [3H]leucine incorporation into rat liver proteins by about 90%: in the case of cycloheximide and pactamycin the effect continued at least over 3 or 4 hours, whereas even high doses of puromycin and fusidic acid were effective not longer than 90–150 minutes. 3. 3. When protein synthesis was inhibited by these antibiotics, [3H]uridine incorporation into three RNA fractions at 0700 and 1100 hours was altered: at 0700 hours, the labeling of the 65° C fraction was increased in all cases. At 1100 hours a relative decrease in the labeling of the 65° C fraction and a relative increase in the 4° C fraction could be observed. Puromycin had no effect at that time of day. 4. 4. Two hours after an injection of cycloheximide at 0700 hours the content of RNA in the nucleus increased, but remained at the normal level at 1500 hours. In both cases a pronounced increase followed by a decrease of the nuclear RNA content occurred within the first 60 minutes. 5. 5. The results support the assumption that protein-associated transport processes or permeabilities are involved in the circadian pattern of RNA labeling.