FIG. 7.

Model for the time and mechanism of action of Prp43p in preribosome biogenesis. Association of Prp43p occurs early, at the 35S stage, in the U3-snoRNP-containing 90S rRNP. In prp43 mutants, the failure of Prp43p to function initiates a discard pathway in which 35S, 27S, and 20S processing defects lead to the degradation of these pre-rRNAs. Red and green spheres indicate transiently interacting large- and small-subunit factors respectively. Dark blue and light blue spheres represent stably interacting large- and small-subunit factors, respectively. A red ring represents Prp43p.