DNA synthesis at a fork in the presence of DNA helicases

In a mixture of Escherichia coli DNA polymerase III holoenzyme, single-strand-binding protein, artificially forked lambda bacteriophage DNA with primer annealed to the leading side of the fork, dNTPs and ATP, DNA synthesis is enhanced by helicase II, less so by helicases, I, III or rep protein of E. coli or T4 phage helicase. The effect of helicase II depends on ATP, it is enhanced by helicase III, and it is not observed using DNA polymerase I or T4 DNA polymerase. In the absence of dNTPs helicase II is less active than helicase I or T4 helicase in unwinding the forked DNA. We believe that helicase II both shifts the forks and stimulates DNA polymerase III. The results support the conclusion derived from previous studies that helicase II is part of the DNA-synthesizing system of E. coli.