The T4 bacteriophage gene 41 protein is the highly processive DNA helicase of the T4 primosome, a central part of the protein machinery that moves the T4 DNA replication fork. The T4 gene 59 protein accelerates the loading of 41 protein onto DNA covered with 32 protein (the T4 single strand binding protein), and it makes the 41 protein DNA helicase activity rapidly available to catalyze replication fork movement through a DNA double helix (Barry, J., and Alberts, B.M. (1994) J. Biol. Chem. 269, 33049-33062). With the aid of the 59 protein, we show that the T4 primosome (the T4 gene 41 and 61 proteins) can move rapidly through a promoter-bound RNA polymerase molecule that would otherwise stop replication fork movement. A second, very different DNA helicase, the T4 dda protein, provides an alternative pathway for replication past this DNA-bound RNA polymerase (Bedinger, P., Hochstrasser, M., Jongeneel, C. V., and Alberts, B.M. (1983) Cell 34, 115-123). Combined with other data, these in vitro experiments allow us to propose a model that explains why either the 59 protein or the dda protein, but not both, are required to begin efficient DNA replication inside the T4 bacteriophage-infected cell.