Sensitivity of low molecular weight RNA synthesis to UV radiation

Abstract

RNA SPECIES C and D (nomenclature of Weinberg and Penman¹) are among the most abundant of the long-lived², homodisperse, low molecular weight nuclear RNAs³, and are present in all vertebrates tested⁴, but their function is unknown. They have several interesting characteristics: they seem to be transcribed from multiple-copy genes (10²–10³ genes per genome)^5,6, to pass through the cytoplasm for a few minutes shortly after their transcription^3,7–10, to have 5′-end caps that are very similar to those of eukaryotic mRNAs^11,12, and to be present in heterogeneous nuclear RNA-protein particles^13,14. Here, we show that C and D RNA syntheses are unexpectedly sensitive to UV radiation in HeLa cells. The effect of UV radiation on RNA synthesis has been used to study the organisation of transcription units¹⁵. UV irradiation, causing random formation of pyrimidine dimers in DNA, results in premature termination of transcription. The probability of causing a UV lesion within a transcription unit is directly proportional to its length, making it possible to measure the distance between a gene and its promoter. The present UV sensitivity data are compatible with the transcription units for the low molecular weight (less than 200-nucleotide long^1,11,12) RNA species C and D being as long as 5 kilobases.