Practice question

The bulk of rRNA nucleotide modifications, pseudouridylation and 2'-O-methylation totaling over 200 sites in human rRNA, occurs in nucleolus, the site of rRNA synthesis and early ribosome assembly. Modification is mediated by small nucleolar RNPs, H/ACA guiding pseudouridine synthase dyskerin and C/D guiding fibrillarin methyltransferase, base-pairing with nascent 45S pre-rRNA co-transcriptionally. Nucleolar concentration of factors couples transcription, modification, folding and protein binding, ensuring modified nucleotides cluster at functional centers like decoding and peptidyl transferase for accuracy and stability. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.