Figure 6. Pol V Concentration as a Function of Time, Following an Instantaneous Pulse of UV at Time Zero, For Different UV Doses.

Below a threshold of about 17 J/m² there is no Pol V and, hence, no mutagenesis. The sharp onset of Pol V, in relation to both time and UV dose, is a direct consequence of the strong heterodimerization of UmuD–UmuD′. Thus, mutagenesis occurs only if the amount of UmuD′ exceeds that of UmuD, so that some free UmuD′ is left to generate Pol V.