5-Fluorouracil substitution alters pre-mRNA splicing in vitro

We are interested in determining whether incorporation of the drug 5-fluorouracil into pre-mRNA can alter RNA processing. The effect of 5-fluorouracil (FUra) substitution on the in vitro splicing of pre-mRNA was studied. 32P-Labeled human beta-globin pre-mRNA containing the first two exons and the first intervening sequence was synthesized in the presence of UTP, FUTP, or both. In vitro splicing reactions generated several abnormal intermediates and products from FUra-substituted transcripts. The appearance of a new minor spliced product was dependent on both the pH of the splicing reaction and the extent of FUra incorporation into pre-mRNA. Abnormal splicing was observed at pH 8.4 and 7.7 but not at pH 6.7. The new minor spliced product was sequenced and found to contain an additional 20 bases derived from the 3'-end of the intervening sequence. The abnormally migrating intermediate had the same structure as the normal lariat-exon intermediate. These results suggest that FUra substitution into pre-mRNA can alter splicing in vitro.