AbstractWe describe a novel biogenesis factor of the 90S pre-ribosome, Puf-A, which is a negative transcriptional target of p53. The expression of Puf-A is not only upregulated in advanced human lung cancer and tumors of patients especially with TP53 mutation, but also is highly prognostic for stage I lung cancer. Loss of Puf-A expression prevents KrasG12D/p53-/-–induced tumor progression in the lungs and induces apoptosis in TP53– mutated cancers and c-Myc/p53-/-–transformed cells as well. Overexpression of Puf-A enhances proliferation of normal cells after c-Myc induction and overcomes the cell-cycle checkpoints incurred by p53 expression. Mechanistically, Puf-A interacts with double-stranded structures of the 5.8S sequence within pre-rRNA and maintains the integrity of 90S pre-ribosomes, thereby impacting early ribosome assembly and export of ribosomes from nuclei. Silencing of Puf-A disrupts the assembly of 90S pre-ribosomes and induces the translocation of its associated nucleophosmin (NPM1) from nucleoli to the nucleoplasm, resulting in impairment of ribosome synthesis. Thus, Puf-A is crucial for over-activation of ribosome biogenesis and contributes to tumor progression and cancer growth.