Figure 8.

Scheme to show possible relationships between complexes. It is likely that sites are selected by affinity and kinetics: the first site to interact with the 5΄ss triggers complex I formation and molecules of U2AF or U2 snRNPs bound elsewhere are displaced. Binding is not limited by any intrinsic process to a single U2AF or U2 snRNP and so several strong sites could be occupied concurrently, as in routes II or III, and the complexes resist displacement. We note that there might be processes linked to transcription in vivo that reduce the chance of additional molecules of U2AF35 binding.