FIGURE 9.

Postulated displacement of domain VI between the two splicing steps. (A) First-step conformation. (B) Second-step conformation. The θ–θ′ and ε–ε′ interactions (Costa et al. 1997a; Toor et al. 2008) are presumed to persist throughout the splicing process, whereas ι–ι′, η–η′, and π–π′ are step-specific interactions, whose location in the Oceanobacillus intron is tentatively inferred from studies on other group II introns (Li et al. 2011b; Robart et al. 2014). A gray ellipse symbolizes the catalytic center of the group II ribozyme and the arrow indicates nucleophilic attack of a phosphodiester bond. Movement of domain VI between its two preferred locations is assumed to occur irrespective of the presence of the 5′ exon (see text).