Interaction of the bacteriophage T4 gene 59 helicase loading protein and gene 41 helicase with each other and with fork, flap, and cruciform DNA

Bacteriophage T4 gene 59 helicase loading protein accelerates the loading o f T4 gene 41 DNA helicase and is required for recombination-dependent DNA r eplication late in T4 phage infection. The crystal structure of 59 protein revealed a two-domain alpha-helical protein, whose N-terminal domain has st rong structural similarity to the DNA binding domain of high mobility group family proteins (Mueser, T. C., Jones, C. E., Nossal, N. G., and Hyde, C, C, (2000) J. Mel. Biol, 296, 597-612), We have previously shown that 59 pro tein binds preferentially to fork DNA, Here we show that 59 protein binds t o completely duplex forks but cannot load the helicase unless there is a si ngle-stranded gap of more than 5 nucleotides on the fork arm corresponding to the lagging strand template. Consistent with the roles of these proteins in recombination, we find that 59 protein binds to and stimulates 41 helic ase activity on Holliday junction DNA, and on a substrate that resembles a strand invasion structure. 59 protein forms a stable complex with wild type 41 helicase and fork DNA in the presence of adenosine 5'-O-(thiotriphospha te). The unwinding activity of 41 helicase missing 20 C-terminal amino acid s is not stimulated by 59 protein, and it does not form a complex with 59 p rotein on fork DNA.