T4 DNA polymerase: Rates and processivity on single-stranded DNA templates

Three different methods have been used to determine the rate at which an individual bacteriophage T4 DNA polymerase molecule moves when synthesizing DNA on a single-stranded DNA template chain. These methods agree in suggesting an in vitro rate for this enzyme of about 250 nucleotides per second at 37 °C. This rate is close to the rate at which bacteriophage T4 replication forks move in vivo (about 500 nucleotides per second). Comparison with the overall amount of DNA synthesis seen in in vitro reactions reveals that only a small fraction of the T4 DNA polymerase molecules present are synthesizing DNA at any one time. This is explicable in terms of the limited processivity of the enzyme in these reactions, along with its capacity for non-productive DNA binding to the DNA template molecules.