Figure 6.

Three-dimensional model of the group I ribozyme core in the region near the U300⋅A97-U277 base triple (7). Dotted lines indicate hydrogen bonds. The interference and phylogenetic data suggest that the base triple is part of a hydrogen bonding network that connects the A97-U277 base pair in helix P3 with U300 in the J8/7 strand and with the C(-5)-G26 base pair in the P1 substrate helix. In this way, U300 within the J8/7 strand mediates the close approach of two helices in the ribozyme active site.