Nucleolin, a major nucleolar protein implicated in preribosome assembly and transcriptional regulation, possesses a C-terminal domain unusually rich in glycine, arginine, and phenylalanine residues. A polypeptide (p10), corresponding to this domain, has been synthesized by means of an Escherichia coli expression system and purified to homogeneity. Nitrocellulose binding assays have clearly shown that this domain of nucleolin is capable of interacting with RNA, and indeed all nucleic acids tested, in an efficient but nonspecific manner. A combination of circular dichroism and infrared spectroscopy provide strong evidence that repeated beta-turns are a major structural component of this polypeptide, which is entirely consistent with its amino acid composition and above all the presence of repeat motifs such as RGGF. Circular dichroism technique also shows that the interaction of p10 with RNA involves an unstacking of the nucleotide bases and an unfolding of the RNA secondary structure. While the role of the C-terminal domain of nucleolin in vivo has yet to be established, our findings suggest that it may act to unfold regions of ribosomal RNA so that a second domain of nucleolin has access to its specific binding site.