Fig. 8.

Pre-2s state vs. the spliceosomal P complex. The location of the 3′ splice site is indicated with arrows. The spliceosomal P complex (PDB accession 6EXN) contains a cleaved 3′ splice site with the green hatched lines indicating the probable backbone direction of the intact splice site given the location of the 3′ intron end and the 3′ exon. In both cases, the 3′ splice site adopts a highly kinked configuration that presents the scissile phosphate to the 5′ exon for nucleophilic attack. The overall configuration of the active sites exhibits structural homology indicating an evolutionarily conserved mechanism for the second step of RNA splicing