The major messenger ribonucleoprotein particle protein p50 (YB-1) promotesnucleic acid strand annealing

p50, a member of the Y-box binding transcription factor family, is tightly associated with eukaryotic mRNAs and is responsible for general translation al regulation. Here we show that p50, in addition to its previously describ ed ability to melt mRNA secondary structure, is capable of promoting rapid annealing of complementary nucleic acid strands. p50 accelerates annealing of RNA and DNA duplexes up to 1500-fold within a wide range of salt concent rations and temperatures. Phosphorylation of p50 selectively inhibits DNA a nnealing. Moreover, p50 catalyzes strand exchange between double-stranded a nd single-stranded RNAs yielding a product bearing a more extended double-s tranded structure. Strikingly, p50 displays both RNA-melting and -annealing activities in a dose-dependent manner; a relatively low amount of p50 prom otes formation of RNA duplexes, whereas an excess of p50 causes unwinding o f double-stranded forms. Our results suggest that the alteration of nucleic acid conformation is a basic mechanism of the p50-dependent regulation of gene expression.