Guided tours: from precursor snoRNA to functional snoRNP

Small nucleolar RNAs (snoRNAs) use base pairing to guide. modification of c onserved nucleotides in functionally important regions of ribosomal RNA The box C/D snoRNAs direct 2'-O-methylation and the box H/ACA snoRNAs direct p seudouridylation. Each snoRNA interacts with proteins, many of them newly i dentified. Progress in understanding how snoRNA sequences are stored within genomes, liberated from precursor molecules and targeted to the nucleolus has begun to elucidate each step in the biogenesis of these critical contri butors to ribosome formation.