Interaction of nucleolin with an evolutionarily conserved pre-ribosomal RNA sequence is required for the assembly of the primary processing complex

The first processing event of the precursor ribosomal RNA (pre-rRNA) takes place within the 5' external transcribed spacer. This primary processing re quires conserved cia-acting RNA sequence downstream from the cleavage site and several nucleic acids (small nucleolar RNAs) and proteins trans-acting factors including nucleolin, a major nucleolar protein, The specific intera ction of nucleolin with the pre-rRNA is required for processing in vitro. X enopus laevis and hamster nucleolin interact with the same pre-rRNA site an d stimulate the processing activity of a mouse cell extract. A highly conse rved 11-nucleotide sequence located 5-6 nucleotides after the processing si te is required for the interaction of nucleolin and processing. In vitro se lection experiments with nucleolin have identified an RNA sequence that con tains the UCGA motif present in the 11-nucleotide conserved sequence, The i nteraction of nucleolin with pre-rRNA is required for the formation of an a ctive processing complex. Our findings demonstrate that nucleolin is a key factor for the assembly and maturation of pre-ribosomal ribonucleoparticles .