Labeling of cytoplasmic liver RNA by (6- 14 C)orotic and 5-fluoro(2- 14 C)orotic acids. Effect of several inhibitors

Orotic and 5-fluoroorotic acid are utilized differentially for the synthesis of the known species of cytoplasmic hepatic RNA. 5-Fluoroorotic acid is incorporated preferentially into a fraction of non-ribosomal RNA which has several properties in common with messenger RNA. The incorporation of both orotate and 5-fluoroorotate into cytoplasmic RNA is influenced by the administration in vivo of several inhibitors but to different degrees. Actinomycin D blocks the incorporation of orotic acid into ribosomal RNA with relatively little effect on the incorporation of 5-fluoroorotate into non-ribosomal RNA. 5-Azacytidine administration leads to an alteration in the incorporation of orotic as well as 5-fluoroorotic acid into cytoplasmic RNA, these effects being dependent on the length of pretreatment with the analogue. 5-Azacytidine given 24 h earlier causes an enhancement of orotic acid incorporation into liver RNA, while no such effect on the labeling of total liver RNA by 5-fluoroorotic acid is seen. Short-term treatment (2 h) with 5-azacytidine leads to the depressed utilization of orotic as well as 5-fluoroorotic acid for RNA synthesis. 5-Azaorotate administration results in an inhibition of the incorporation of orotic and 5-fluoroorotic acid into total liver RNA without affecting their utilization during later phases of the treatment. The observed differences in the incorporation of labeled orotic and 5-fluoroorotic acid into individual fractions of hepatic cytoplasmic RNA are discussed in relation to their known labeling characteristics.