Nucleolin promotes secondary structure in ribosomal RNA. 
The effect of nucleolin on the secondary structure of RNA was studied using circular dichroism (CD).  Nucleolin caused decreases in the main positive bands and shifts to higher wavelengths in the CD spectra of synthetic polynucleotides such as poly(G) and poly(A) indicating helix destabilizing activity.  In contrast, nucleolin effected increases in signal and shifts to lower wavelengths of the peaks of CD spectra of ribosomal RNA, suggesting enhancement of secondary structure.  Another major nucleolar RNA binding protein, B23, had helix destabilizing activity but did not enhance RNA secondary structure.  It is proposed that nucleolin promotes formation of secondary structure in preribosomal RNA during the early stages of ribosome biogenesis.