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Many RNA nucleases and helicases participate in ribosome biogenesis, but how they cooperate with each other is largely unknown. Here we report that in vivo cleavage of the yeast pre-rRNA at site D, the 3′-end of the 18 S rRNA, requires functional interactions between PIN (PilT N terminus) domain protein Nob1 and the DEAH box RNA helicase Prp43. Nob1 showed specific cleavage on a D-site substrate analogue in vitro, which was abolished by mutations in the Nob1 PIN domain or the RNA substrate. Genetic analyses linked Nob1 to the late pre-40 S-associated factor Ltv1, the RNA helicase Prp43, and its cofactor Pfa1. In strains lacking Ltv1, mutation of Prp43 or Pfa1 led to a striking accumulation of 20 S pre-rRNA in the cytoplasm due to inhibition of site D cleavage. This phenotype was suppressed by increased dosage of wild-type Nob1 but not by Nob1 variants mutated in the catalytic site. In ltv1/pfa1 …