Significance Ribosomes are essential cellular nanomachines responsible for all protein synthesis in vivo. Efficient and faithful ribosome biogenesis requires a plethora of assembly factors whose precise role and timing of action remains to be established. Here we determined the crystal structure of Bud23–Trm112, which is required for efficient pre-rRNA processing steps leading to 18S rRNA synthesis and methylation of 18S rRNA at position G1575. For the first time, to our knowledge, we identified where on Bud23–Trm112 the contacts with precursor ribosomes occur. We further report that the essential helicase Dhr1 interacts directly with Bud23–Trm112, proposing a concerted action of these proteins in ribosome assembly. Finally, we reveal that the methyltransferase activity of Bud23–Trm112 and its requirement for pre-rRNA processing are disconnected in time.