SCHEME 2.

Minimal model for initiation kinetics. We model the kinetics of initiation starting with RNA (R) binding to enzyme (E). Then in sequential steps nucleotides (not shown) bind and react to form primers of varying lengths (p2, p3, etc.). The fitting derives apparent rates of reaction for each step in the model, which in reality represents at least two steps. Therefore, the model is used only to capture the essential features of the mechanism and kinetics of initiation rather than defining intrinsic rate constants, which is not possible given the information content of the data relative to the minimal number of steps in the model.