The catalytic core of the spliceosome
The spliceosome is composed of five small nuclear RNAs and about a hundred of proteins, whose function is to recognize and splice exons together. A longstanding question about spliceosome function is whether snRNAs or proteins catalyze the splicing reaction. Since the spliceosome is a metalloenzyme, finding the spliceosomal components that coordinate catalytic metals is crucial in addressing the question. There is evidence of genetic and biochemical nature indicating that the U6 snRNA is at the catalytic core of the spliceosome.
By reconstituting the yeast spliceosome with a phosphorothioate-substituted U6 snRNA, we demonstrated that the 5’ phosphate of the eightieth nucleotide (U80) of U6 coordinates a catalytically important magnesium ion for the first step of splicing. We found, using similar approaches, that phosphates of A59 and G60 of U6 snRNA are also critical for the catalytic activity of the spliceosome. Thus, these observations support the idea that U6 snRNA is not only at the core of the spliceosome but also directly participates in the chemical reaction of splicing, and that the spliceosome may have a evolutionary root in RNA-based machinery.