Shift of the Noc2-TAP particle populations upon Rlp7 depletion and treatment with LiCl. (A) Scheme depicting how the population of Noc2 containing pre-ribosomal particles (WT: wild-type conditions, black line) is shifted to earlier stages either due to depletion of Rlp7 (ΔRlp7, green line) or a combination of Rlp7 depletion and LiCl treatment (ΔRlp7 + LiCl, purple line) (Fig. 5). (B) Under wild-type conditions, pre-rRNA is processed in the order A0 → A1 → A2 → A3 (A2 pathway) leading to a pre-40S subunit associated with 20S pre-rRNA. Perturbations in the first pre-rRNA processing steps cause the cells to switch to the A3 pathway, which allows cleavage at site A3 before the other processing steps (A3 pathway), resulting in a pre-40S subunit associated with 23S pre-rRNA (and its downstream processing intermediates; Supplemental Fig. S8B). A block in processing at site A3 triggers a feedback loop that prevents processing at site A2, resulting in the formation of aberrant 22SE pre-rRNA (or 24S pre-rRNA if sites A0 and A1 are not processed) (Figs. 4 and 5).
