Nucleolin functions in the first step of ribosomal RNA processing.

The first processing step of precursor ribosomal RNA (pre-rRNA) involves a cleavage within the 5' external transcribed spacer. This processing requires sequences downstream of the cleavage site which are perfectly conserved among human, mouse and Xenopus and also several small nucleolar RNAs (snoRNAs): U3, U14, U17 and E3. In this study, we show that nucleolin, one of the major RNA-binding proteins of the nucleolus, is involved in the early cleavage of pre-rRNA. Nucleolin interacts with the pre-rRNA substrate, and we demonstrate that this interaction is required for the processing reaction in vitro. Furthermore, we show that nucleolin interacts with the U3 snoRNP. Increased levels of nucleolin, in the presence of the U3 snoRNA, activate the processing activity of a S100 cell extract. Our results suggest that the interaction of nucleolin with the pre-rRNA substrate might be a limiting step in the primary processing reaction. Nucleolin is the first identified metazoan proteinaceous factor that interacts directly with the rRNA substrate and that is required for the processing reaction. Potential roles for nucleolin in the primary processing reaction and in ribosome biogenesis are discussed.