The beta protein of phage λ promotes strand exchange

Abstract

Bacteriophage λ encodes a 28 kDa protein called β that binds to single-stranded DNA and promotes the renaturation of complementary single strands. β Protein fails to bind directly to duplex DNA but remains bound to the DNA product of renaturation that β itself catalyzes. These observations led to an examination of the ability of β protein to promote strand exchange. β Protein caused the replacement of a 43-mer oligonucleotide annealed to M13 circular single-stranded DNA by a homologous 63-mer whose 20 extra nucleotide residues were complementary to the adjacent 3' region of M13 DNA. The role of β protein in this reaction was manifested in several ways: β protein pushed the exchange through four to eight mismatches, which blocked exchange mediated by spontaneous renaturation and branch migration; β imposed a polarity on the strand exchange that was lacking in the spontaneous reaction; and β remained bound to the heteroduplex product of strand exchange. These observations reveal a mechanism by which a protein can drive strand exchange in one direction without using ATP or any other exogenous source of energy.