THE BETA-PROTEIN OF PHAGE-LAMBDA PROMOTES STRAND EXCHANGE

Bacteriophage lambda encodes a 28 kDa protein called beta that binds t o single-stranded DNA and promotes the renaturation of complementary s ingle strands. beta Protein fails to bind directly to duplex DNA but r emains bound to the DNA product of renaturation that beta itself catal yzes. These observations led to an examination of the ability of beta protein to promote strand exchange. beta Protein caused the replacemen t of a 43-mer oligonucleotide annealed to M13 circular single-stranded DNA by a homologous 63-mer whose 20 extra nucleotide residues were co mplementary to the adjacent 3' region of M13 DNA. The role of beta pro tein in this reaction was manifested in several ways: beta protein pus hed the exchange through four to eight mismatches, which blocked excha nge mediated by spontaneous renaturation and branch migration; beta im posed a polarity on the strand exchange that was lacking in the sponta neous reaction; and beta remained bound to the heteroduplex product of strand exchange. These observations reveal a mechanism by which a pro tein can drive strand exchange in one direction without using ATP or a ny other exogenous source of energy. (C) 1998 Academic Press Limited.