Different Small Nuclear Ribonucleoprotein Particles Are Involved in Different Steps of Splicing Complex Formation

Splicing of the introns in messenger RNA precursors (pre-mRNA) is an important part of the chain of post-transcriptional processing reactions that these primary transcripts have to undergo in the nuclei of eukaryotic cells (for reviews, see Padgett et al. 1986; Green 1986). Conceptually, all known types of RNA splicing can be subdivided into several distinct steps: Specific internal sequences must be recognized and the molecules must be folded such that distant splice sites are brought into proximity and become precisely aligned. This is a prerequisite for the ensuing catalysis of the two-step cleavage-ligation reactions that give rise to spliced RNA. In the case of self-splicing RNAs (for reviews, see Cech 1986, 1987; Cech and Bass 1986; Been et al., this volume), recognition-alignment, as well as the provision of an RNA catalyst, is accomplished by the folding of the molecules via intramolecular base pairing into specific structures.