RNA synthesis and stability in UV-irradiated and nonirradiated mouse L cells

In mouse L cells, relatively low doses of UV light (e.g., about 35 J/m2) induced the rapid breakdown of the molecules of many RNA species transcribed shortly before irradiation. This included 28S, 18S, 5.8S, and 5S rRNA, U1, U2, U3, U4, and U5 small nuclear RNA, but not the main band of transfer RNAs or 7SL RNA. At higher UV doses, an RNA band that contains tRNAleu was also degraded rapidly after UV irradiation. RNA molecules synthesized long before irradiation (e.g., 22 h for small RNAs, 4 h for large rRNAs) were not affected. Our results suggest that the maturation and/or assembly into fully mature ribonucleoprotein particles of several small RNA species is not completed 4 h after transcription. The effect of UV radiation occurred in mouse L cells, but not in human HeLa or KB cells. In a previous report, L cells were transformed by DNA transfection with two mouse U1b RNA genes, named U1.1 and U1.2. We observed now that, in L cells transformed with the U1.2 gene, the ratio of radioactivity in the apparent U1b and U1a RNA precursors after 5 min of labeling was about 20 times higher than a) this ratio in briefly labeled L cells that had been transformed with the U1.1 gene, and b) the ratio of radioactive mature U1b and U1a RNA after 20 h of chase in L cells transformed with the U1.2 gene. These results suggest that very high levels of U1b RNA are transcribed from the exogenous U1.2 gene copies, followed by the rapid degradation of most of these transcripts.