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. 1999 Aug 1;13(15):1983–1993. doi: 10.1101/gad.13.15.1983

Figure 6.

Figure 6

A possible model of interactions at the first and the second step of splicing. At the first step of the reaction the 5′SS is properly positioned for the nucleophilic attack (arrow) by the branch site A; the 3′SS is not required at the catalytic center. The carboxy-terminal segment of Prp8 interacts with the 5′SS at this stage, as indicated by a number of biochemical studies. Before or during the second step of splicing both 5′SS and 3′SS are positioned in close proximity of each other, the ACAGA box of U6 snRNA, and the carboxy-terminal segment of Prp8 described in this paper. The proximity of all these elements at the catalytic center may explain the suppressor effect of the same prp8 alleles on both the 5′SS and 3′SS mutations during the second step of splicing. Noncanonical interactions: G-G between the terminal intron residues, U-A between the second (U+2) intron residue, and A51 of U6 snRNA are indicated by short white bars. In addition, Watson–Crick interactions between U6 and U2 snRNA (helix Ia, Ib), branch site and U2 snRNA, and the 5′SS and U6 snRNA are indicated.