Protein displacement by an assembly of helicase molecules aligned along single-stranded DNA.

Helicases are molecular motors that unwind double-stranded DNA or RNA. In addition to unwinding nucleic acids, an important function of these enzymes seems to be the disruption of protein-nucleic acid interactions. Bacteriophage T4 Dda helicase can displace proteins bound to DNA, including streptavidin bound to biotinylated oligonucleotides. We investigated the mechanism of streptavidin displacement by varying the length of the oligonucleotide substrate. We found that a monomeric form of Dda catalyzed streptavidin displacement; however, the activity increased when multiple helicase molecules bound to the biotinylated oligonucleotide. The activity does not result from cooperative binding of Dda to the oligonucleotide. Rather, the increase in activity is a consequence of the directional bias in translocation of individual helicase monomers. Such a bias leads to protein-protein interactions when the lead monomer stalls owing to the presence of the streptavidin block.