AFs binding to 25S rRNA domain V and r-proteins surrounding the PET exit are present in decreased levels in B mutants. (A) The B-factors Nsa2 and Nog2 and the AFs Rsa4, Nug1, and Nop53 bind to 25S rRNA domain V. (B) As assembly proceeds, the globular bodies of the r-proteins L19, L25, L31, L17, L26, L35, and L37 form a platform surrounding the exit of the PET to which Arx1 can then bind. Structures shown are PDB IDs 6EM1, 6ELZ, and 3JCT.