HELA NUCLEAR-PROTEIN RECOGNIZING DNA TERMINI AND TRANSLOCATING ON DNA FORMING A REGULAR DNA MULTIMERIC PROTEIN COMPLEX

Employing an exonuclease III protection assay we detected a protein in crude HeLa nuclear extracts binding, with apparent sequence specificity, to molecular ends of adenovirus type 2 (Ad2) DNA. This protein, designated nuclear factor IV (NFIV), was purified to homogeneity and was shown to be a hetero-dimer of 72,000 and 84,000 Mr. Binding to terminal Ad2 sequences was strongly enhanced by the presence of either of the sequence-specific DNA-binding proteins nuclear factor I and nuclear factor III. These proteins appeared to function as blockades for translocation of NFIV on DNA, thus producing apparent sequence specificity. In the absence of such a blockade, NFIV moved freely, without energy input, on any double-stranded DNA forming a regular DNA-multimeric protein complex as shown by methidiumpropyl EDTA footprinting and electron microscopy. Binding is completely dependent upon the presence of molecular ends. Evidence was obtained for a two-step mechanism in which termini are recognized by NFIV and used as a starting point for subsequent translocation. The possible functions of the protein in adenovirus DNA replication and in cellular processes requiring DNA termini are discussed.