Figure 5. Packaging Models that Produce a Non-Integer Step Size.
(a) Depiction of a translocation model in which all subunits eventually contact the DNA (cyan spheres.) The contacting subunit is outlined in black. (b) In such a model the size of internal conformational changes set the step size (side view.) (c) Depiction of a translocation model in which only two subunits contact the DNA (black outline.) (d) In such a model, one subunit maintains contact with the DNA (the latch) while the loading of each ATP introduces relative subunit-subunit rotations which distort the ring. This distortion extends one subunit (the lever) along the DNA by ∼10-bp. The DNA contact point is then transferred from the latch to the lever, and the release of hydrolysis products relaxes the ring, retracting the lever and the DNA. The DNA contact is then transferred back to the latch, the ring resets, and the cycle begins again. Because there are four subunits, the ring is retracted in four steps, dividing a 10-bp step into four ∼2.5-bp substeps. The subunit color scheme is the same as in Figure 4.