AbstractAssembly of ribosomal subunits occurs via parallel pathways, which accelerate the process and render it more robust. Nonetheless, in vitro analyses have also demonstrated that some assembly pathways are dead-ends, presumably due to rRNA misfolding. If and how these non-productive pathways are avoided during assembly in vivo remains unknown. Here we use a combination of biochemical, genetic, proteomic and structural analyses to demonstrate a role for assembly factors in biasing the folding landscape away from non-productive intermediates. By binding Rrp36, Rrp5 is prevented from forming a premature interaction with the platform, which leads to a dead-end intermediate, and a misassembled platform that is functionally defective. The DEAD-box ATPase Has1 separates Rrp5 and Rrp36, allowing Rrp5 to reposition to the platform, thereby promoting ribosome assembly and enabling rRNA processing. Thus, Rrp36 establishes an ATP-dependent regulatory point that ensures correct platform assembly by opening a new folding channel that avoids funnels to misfolding.