Effects of ultraviolet irradiation and postirradiation incubation on heterogenous nuclear RNA size in murine cells.

We have analyzed the decrease in synthesis of individual size classes of heterogeneous nuclear RNA (hnRNA) in ultraviolet (UV)-irradiated Merwin plasmacytoma (MPC-11) cells at various times of postirradiation incubation. HnRNA from nonirradiated control cells is distributed over a wide range from approximately 60S to 5S, with 42S RNA carrying more label than any other size class. HnRNA from UV-irradiated cells shows a dose-dependent shift in size distribution toward lower molecular weight. The size distribution of hnRNA synthesized after prolonged times of postirradiation incubation is restored toward normal, i.e., synthesis of long RNA molecules increases relative to the synthesis of short ones. Analysis of the total number of hnRNA chains synthesized during a 20-min [(3)H]uridine pulse shows a considerable reduction in their number with increasing UV dose. Murine cell lines are excision-repair-deficient but capable of post replication repair inhibited by caffeine. HnRNA transcripts of cells incubated in its presence were studied. The caffeine, which has no effect on hnRNA size in control cells, inhibits to a considerable extent the restoration of full-length transcripts during postirradiation incubation. The lack of excision repair in MPC-11 was confirmed by the analysis of pyrimidine dimers in trichloracetic acid-insoluble and soluble fractions within 8 h of postirradiation incubation.The size of parental and daughter strand DNA in UV-irradiated cells was correlated with RNA transcript size. The parental DNA in these experiments does not change its size as a consequence of UV exposure and postirradiation incubation. In contrast, daughter DNA strands are short in UV-irradiated cells and they increase in size during postirradiation incubation to reach the size of parental strands after 8 h.