It has been shown previously that the synthesis of small nuclear RNAs (snRNAs) Ul, U2, U3, U4, and U5, in contrast to that of all other RNA species tested, decreases markedly within 2 h of cell incubation after exposure to UV light (254 nm), while pyrimidine dimers are being removed from DNA. We examined the possibility that the postirradiation cell incubation-dependent, UV light-induced inhibition of snRNA synthesis might reflect hypersensitivity of the snRNA transcriptional domains to single-stranded DNA nicks or relaxation of DNA torsional stress or both that occur during DNA repair. This late suppression of snRNA biosynthesis was as pronounced in UV light-irradiated (DNA incision-deficient) xeroderma pigmentosum fibroblasts (complementation group A) as in irradiated normal human fibroblasts. The synthesis of snRNAs was not preferentially sensitive to -y radiation (which produces single-stranded DNA breaks) or novobiocin or nalidixic acid (which induce DNA relaxation). Neither of these two drugs prevented the UV light-induced inhibition of snRNA synthesis observed during postirradiation cell incubation. These results suggest that the late suppression of snRNA synthesis does not result from hypersensitivity of snRNA transcriptional domains to single-stranded DNA cleavages or relaxation of DNA torsional strain. The UV light-induced late inhibition of snRNA synthesis: (i) shows an inactivation curve whose slope differs from that observed immediately after irradiation; (ii) is seen in untransformed cells as well as established cells lines; and (iii) has been conserved between birds and mammals.The size of transcription units can be measured by UV transcription mapping (24). UV light induces the random dimerization of adjacent pyrimidine pairs along a DNA strand, interrupting the elongation of transcription. There are three unexpected features about the effect of UV radiation on the biosynthesis of small nuclear RNA (snRNA) species Ul, U2, U3, U4, and U5. First, although Ul through U5 snRNAs and their known precursors are very short (1, 5), their biosynthesis is very sensitive to UV light (3, 6). In contrast, for all other RNA species tested there is a strict correlation between the length of the transcription unit and the UV light sensitivity of its expression (24). The term early inhibition will be used here for the UV light-induced suppression of snRNA biosynthesis that is easily seen after relatively high UV doses and very short postirradiation cell incubations (for example, about 290 J/m2 and no more than 10 min, respectively). Second, a small fraction of Ul, U2, U3, and U5 snRNA biosynthesis remains unaffected at high UV doses when cell radiolabeling begins soon (within 10 min) after irradiation (6). The presence of a UV lightresistant subpopulation of RNA synthesis has not been observed in any other RNA species whose synthesis is inhibited by UV radiation. Third, the suppression of Ul through U5 snRNA synthesis increases within 2 h of postirradiation cell incubation (6). This has not been seen in any other ...

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It has been shown previously that the synthesis of small nuclear RNAs (snRNAs) U1, U2, U3, U4, and U5, in contrast to that of all other RNA species tested, decreases markedly within 2 h of cell incubation after exposure to UV light (254 nm), while pyrimidine dimers are being removed from DNA. We examined the possibility that the postirradiation cell incubation-dependent, UV light-induced inhibition of snRNA synthesis might reflect hypersensitivity of the snRNA transcriptional domains to single-stranded DNA nicks or relaxation of DNA torsional stress or both that occur during DNA repair. This late suppression of snRNA biosynthesis was as pronounced in UV light-irradiated (DNA incision-deficient) xeroderma pigmentosum fibroblasts (complementation group A) as in irradiated normal human fibroblasts. The synthesis of snRNAs was not preferentially sensitive to gamma radiation (which produces single-stranded DNA breaks) or novobiocin or nalidixic acid (which induce DNA relaxation). Neither of these two drugs prevented the UV light-induced inhibition of snRNA synthesis observed during postirradiation cell incubation. These results suggest that the late suppression of snRNA synthesis does not result from hypersensitivity of snRNA transcriptional domains to single-stranded DNA cleavages or relaxation of DNA torsional strain. The UV light-induced late inhibition of snRNA synthesis: shows an inactivation curve whose slope differs from that observed immediately after irradiation; is seen in untransformed cells as well as established cells lines; and has been conserved between birds and mammals.