Inhibition and recovery of ribosomal RNA synthesis in ultraviolet-irradiated mammalian cells

Abstract

The effects of ultraviolet light on rRNA synthesis were studied in stationary, monkey kidney CV-1 cells. Separate cultures were irradiated with a single ultraviolet dose of up to 525 erg/mm 2 at different time intervals before incubation with [ 3H]uridine. The labeled RNA prepared from whole cells was analyzed by polyacrylamide-agarose gel electrophoresis. In cells ultraviolet irradiated 30 min before transcription analysis, the rate of total RNA and of 45 S RNA synthesis are depressed whereas a synthesis of unusual RNAs, shorter than 45 S RNA, appears. The rates of 28 S and of 18 S RNA synthesis decrease as a function of ultraviolet dose and at the same time the ratio of 18 S to 28 S RNA increases. These results are consistent with the idea of a premature RNA chain termination on ultraviolet-damaged DNA. The ultraviolet-induced alterations of transcription disappear progressively with time after irradiation: the rates of total RNA and of 45 S RNA synthesis increase whereas the synthesis of unusual RNAs decreases; in parallel the rates of 28 S and 18 S RNA synthesis recover with a shift of the 18 S to 28 S RNA ratio toward the normal value 1 : 1, probably as a direct consequence of the repair of ultraviolet-induced lesions in the DNA. The kinetics of recovery are dose dependent and are comparable to those of the excision-repair of DNA photoproducts as measured by unscheduled DNA synthesis in ultraviolet-irradiated CV-1 cells. Fluorodeoxyuridine does not affect recovery. The data also show that in CV-1 cells, rRNA genes are arranged after their promotor in the order 18 S rRNA – 28 S rRNA.