Exploring structural determinants and the role of nucleolin in formation of the long-range interaction between untranslated regions of p53 mRNA.

p53 protein is a key regulator of cellular homeostasis by coordinating framework of anti-proliferative pathways as a response to various stress factors. Although the main mechanism of stress-dependent induction of p53 protein relies on posttranslational modifications influencing its stability and activity, a growing number of evidences suggest that complex regulation of p53 expression occurs also at the mRNA level. This study explore structural determinants of long-range RNA-RNA interaction in p53 mRNA, crucial for stress-dependent regulation of p53 protein translation. We demonstrate that the eight nucleotide bulge motif plays a key structural role in base pairing of complementary sequences from the 5' and 3' untranslated regions of p53 mRNA. We also show that one of the p53 translation regulators, nucleolin, displays an RNA chaperone activity and facilitates the association of sequences involved in the formation of long-range interaction in p53 mRNA. Nucleolin promotes base pairing of complementary sequences through the bulge motif since mutations of this region reduce or inhibit pairing while compensatory mutations restore this interaction. Mutational analysis of nucleolin reveal that all four RNA recognition motifs are indispensable for optimal RNA chaperone activity of nucleolin. These observations help to decipher the unique mechanism of p53 protein translation regulation pointing bulge motif and nucleolin as the critical factors during intramolecular RNA-RNA recognition in p53 mRNA.