rRNA chemical modifications mainly occur in
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 transferas
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6, Nucleolar snoRNP-guided rRNA modifications
