Model of BR mRNP acquisition of EJC core, NXF1, UPF2, and UPF3 in relation to transcription, processing, and export. (a) At the BR gene (gray line), premRNPs (light blue) are assembled, and the premRNAs are largely spliced. In connection with splicing, the EJC core (purple circles) becomes associated with the premRNP. The EJC core remains associated with the mRNP in the interchromatin and during export. (b) New BR mRNPs (light blue) are released into the interchromatin and move in all directions. (c) They become part of an interchromatin pool of BR mRNPs. In the interchromatin, the BR mRNPs bind NXF1, UPF2, and UPF3 (yellow and orange crosses). At a given moment 10–25% of the BR mRNPs contain these proteins. The pool contains old BR mRNPs (dark blue, boxed by dark blue line) and new BR mRNPs (light blue, boxed by light blue line). NXF1, UPF2, and UPF3 bind to both old and new BR mRNPs. Some BR mRNPs have recruited the proteins at an earlier time point (orange crosses), and some BR mRNPs have just recruited the proteins (yellow crosses). In steady state, the pool receives newly synthesized BR mRNPs and loses an approximately equal number through export to the cytoplasm. Both new and old BR mRNPs are exported if they contain NXF1, UPF, and UPF3 (boxed by black line). (d) BR mRNPs that are exported are a mixture of those that have contained the proteins for some time and those that have just obtained them. (e) BR mRNPs that do not yet contain NXF1, UPF2, and UPF3 can diffuse and interact with the basket of the NPC, but they are rejected and return into the interchromatin.