Recycling of DNA Polymerases at the Replication Fork of Bacteriophage T7

T7 gene 5 protein (gp5) is distributive DNA polymerase. When bound to its processivity factor, Escherichia coli thioredoxin (trx), its processivity is increased from few nucleotides to few hundreds of nucleotides. In the presence of the T7 gene 4 helicase (gp4), the processivity of gp5/trx on duplex DNA increases to thousands of nucleotides. During such processive DNA synthesis on duplex DNA we find that DNA polymerase in solution can exchange with the active DNA polymerase via a mechanism that does not affect either the rate or processivity of replisome. In these studies we have made use of a T7 gp5 in which tyrosine 526 is replaced with phenylalanine (gp5‐Y526F). The altered polymerase discriminates against the chain terminating dideoxynucleotides thus providing a probe for the exchange reaction. Using a genetically altered gp5/trx that can not interact with the C‐terminus of the helicase we show that the exchange of polymerase is mediated by the interaction of polymerases for solution with gp4. We postulate that the hexameric gp4 not only stabilizes the polymerase at the replication fork but that it also serves as a reservoir for DNA polymerases in the event that the functioning polymerase dissociates.