Association of U6 snRNA with the 5'-splice site region of pre-mRNA in the spliceosome.

U6 snRNA is one of the five RNA species required for splicing of nuclear pre-mRNAs. High conservation of its sequence has led to the hypothesis that U6 snRNA plays a catalytic role in splicing. If this is the case, U6 snRNA should be localized close to sites where the splicing reaction occurs. However, this has never been demonstrated. Here, we have shown that U6 snRNA is cross-linked to the 5'-splice site region of pre-mRNA by UV irradiation during the in vitro splicing reaction. We have also detected the cross-link of U6 snRNA and the region around the branchpoint of the intron lariat. The results show that U6 snRNA is present near the splice sites in the splicing reaction and support the idea that U6 snRNA is a catalytic element in the spliceosome.