  DNA polymerases are an essential class of enzymes or molecular motors that
catalyze processive DNA syntheses during DNA replications. A critical issue for
DNA polymerases is their molecular mechanism of processive DNA replication. We
have previously proposed a model for chemomechanical coupling of DNA
polymerases, based on which the predicted results have been provided about the
dependence of DNA replication velocity upon the external force on Klenow
fragment of DNA polymerase I. Here, we performed single molecule measurements
of the replication velocity of Klenow fragment under the external force by
using magnetic tweezers. The single molecule data verified quantitatively the
previous theoretical predictions, which is critical to the chemomechanical
coupling mechanism of DNA polymerases. A prominent characteristic for the
Klenow fragment is that the replication velocity is independent of the
assisting force whereas the velocity increases largely with the increase of the
resisting force, attains the maximum velocity at about 3.8 pN and then
decreases with the further increase of the resisting force.
