We are studying some apparently novel effects of UV radiation, a major environmental carcinogen and mutagen, on animal cells. Our main goals are to determine the detailed molecular mechanisms underlying three unexpected observations made in this laboratory about the effect of UV light on the synthesis of small nuclear RNA (snRNA) species U1, U2, U3, U4 and U5 (U snRNAs are required for processing messenger RNA precursors): a) there is an immediate, high sensitivity of snRNA transcription to UV light, although the known snRNA precursors are very short (instead, the transcription of only RNA species generated from long primary transcripts is sensitive to UV light); b) immediately after UV irradiation, a small fraction of U1, U2, U3 and U5 RNA synthesis is unaffected by (even high doses of) UV light (in contrast, in the case of RNA species whose synthesis is sensitive to UV radiation, that synthesis is fully sensitive); and c) there is a delayed UV-induced inhibition of snRNA transcription, that requires UV light fluences that are considerably lower than those needed for a comparable level of immediate inhibition, that appears to be a reaction distinct from the early inhibition, and that has not been observed with any other RNA species. Our work will focus primarily on the expression of the mammalian genes for U1 RNA, since more is known about them. We propose some hypotheses and suggest that they can be tested by correlation of the expression and structure of U1 RNA genes after UV irradiation. The expression studies (assays of U1 RNA synthesis) include: a) mammalian cells transformed with U1 genes by DNA transfection; b) transcription in isolated nuclei; c) microinjection of human U1 genes into Xenopus laevis oocyte nuclei; d) transcription of U1 genes in whole cell extracts; and e) transient expression of U1 genes transfected into mammalian cells. The structural studies of U1 RNA genes and their flanking regions involve analysis of DNA and chromatin. The suppression of RNA transcription, but not of RNa processing, is known to be an early effect of UV irradiation. In contrast, our results suggest that the maturation, but not the transcription, of U6 snRNA is inhibited shortly after UV irradiation in KB cells. A secondary goal of this project is to test two proposed models of the mechanism of this inhibition. These studies should provide important information about the mechanisms of: a) expression of the unique group of (RNA polymerase II) U snRNA transcription units; and b) some apparently novel effects of UV radiation on animal cells.
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