Secondary structure of the spliceozyme, based on the group I intron ribozyme from Tetrahymena.

(A) Before the reaction, the substrate consists of two exons (red) and an intervening intron (blue). The spliceozyme (black) uses its 5′-terminus to form the P1 duplex, the P1 extension (P1ex), and the 5′-duplex to position the substrate 5′-splice site (filled triangle). The ribozyme 3′-terminus forms the P9.2 duplex and the P9.0 duplex to position the substrate 3′-splice site (empty triangle). (B) After two transphosphorylation reactions the 5′-exon and the 3′-exon of the substrate are joined. The 5′-terminus of the intron is capped by an exogenous G (black) and the 3′-terminus of the intron is liberated. The sequences correspond to the removal of an intron from splice site 258 of the CAT pre-mRNA used in this study. The internal sequence of the intron is drawn as a bold blue line. Note that a hairpin terminator is added to the 3′-terminus for all reactions in cells. The secondary structure of the ribozyme is based on [58] with the alteration that the P4-P6 domain was positioned on the right side for clarity.