Proposed role of MRB1 core assembly in RNA editing. (A) Assembly of editing machinery on duplexed gRNA:mRNA in the native state of untreated T. brucei. 3′-Oligo(U) bearing gRNA strand on top with 3′-poly(A/U) tail appended, partially edited mRNA on bottom. Step I: 20 S RECC assembled with gRNA-bound GAP1/2 and MRB1 core as well as TbRGG2 subcomplex bound to mRNA to form 40 S particle. Arrows indicate conformational change removing GAP1/2 from gRNA to allow RECC access to editing site as defined by gRNA:mRNA duplex. Step II: RECC processes mRNA:gRNA free of obstructive GAP1/2 heterotetramer. Step III: gRNA is degraded after use in editing of prior mRNA/gRNA duplex. RECC and GAP1/2 assumed to disassociate. Question mark (?) indicates that MRB1 core and TbRGG2 remaining attached to still partially edited mRNA remains unknown. (B) Consequence of MRB1 core disassembly (dashed outline) caused by MRB8620 silencing on editing. Crossed arrows indicate that the conformational change freeing GAP1/2 from the gRNA:mRNA duplex, allowing RECC to bind to the hybridized transcripts, does not occur because of disassembly of MRB1 core.
