(A) Cryo-EM density maps of nucleolar states pre-A1, pre-A1*, and post-A1 at 3.6, 3.9, and 2.7 Å, respectively. Modules, assembly factors, and RNA are labeled, and compositional changes are indicated at the bottom. (B) Atomic models of the three states displayed and labeled as in (A). (C) Structures of RNA elements in each state. 5′ domain, green; central domain, beige; 3′ major domain, gray; 3′ minor domain, light pink; 5′ ETS, yellow; U3 snoRNA, pink.