Recent experiments have measured the rate of replication of DNA catalyzed b
y a single enzyme moving along a stretched template strand. The dependence
on tension was interpreted as evidence that T7 and related DNA polymerases
convert two (n = 2) or more single-stranded template bases to double helix
geometry in the polymerization site during each catalytic cycle. However, w
e find structural data on the T7 enzyme-template complex indicate n = 1. We
also present a model for the "tuning" of replication rate by mechanical te
nsion. This model considers only local interactions in the neighborhood of
the enzyme, unlike previous models that use stretching curves for the entir
e polymer chain. Our results, with n = 1, reconcile force-dependent replica
tion rate studies with structural data on DNA polymerase complexes.