Cms1 coordinates stepwise local 90S pre-ribosome assembly with timely snR83 release Table S2

Ribosome synthesis begins in the nucleolus with 90S pre-ribosome construction, but little is known about how the many different snoRNAs that modify the pre-rRNA are timely guided to their target sites. Here, we report a role for Cms1 in such a process. Initially, we discover CMS1 as null suppressor of a nop14 mutant impaired in Rrp12–Enp1 factor recruitment to the 90S. Further investigations detect Cms1 at the 18S rRNA 3'-major domain of an early 90S that carries H/ACA snR83, known to guide pseudouridylation at two target sites within the same subdomain. Cms1 co-precipitates with many 90S factors, but Rrp12-Enp1 encircling the 3'-major domain in the mature 90S is decreased. We suggest that Cms1 associates with the 3’-major domain during early 90S biogenesis to restrict premature Rrp12–Enp1 binding, but allows snR83 to timely perform its modification role before the next 90S assembly steps coupled with Cms1 release take place.