Nucleolin promotes homologous DNA pairing in vitro

We purified to near homogeneity a previously identified 100 kDa mammalian h omologous DNA pairing protein. The purified 100 kDa protein also catalyzed high levels of cell-free homologous DNA recombination activity. This ATP-de pendent activity Mas capable of forming conservative recombinant products b etween two circular double-stranded DNA molecules. We were unable to detect any DNA polymerase, DNA ligase, or 5' or 3' exonuclease activity associate d with this purified material. The purified 100 kDa protein bound silver ni trate ns well as a monoclonal antibody specific for nucleolin. A recombinan t protein comprised of the Escherichia coli maltose-binding protein fused t o the carboxyl-terminal two-thirds of human nucleolin possessed homologous DNA pairing activity. These data indicate that the 100 kDa homologous DNA p airing protein is nucleolin. The observation that nucleolin can carry out h omologous DNA strand pairing in vitro raises the prospect that it may funct ion similarly in vivo.