FIG. 4.

Hypothetical model explaining roles of YabJ and PRPP. PRPP is required to inhibit binding of PurR, leading to state I. The PRPP concentration in the cell is increased by Guo and decreased by Ade. Basal activity results from partial occupancy of the control region with PurR. Repression requires the binding of additional molecules of PurR (state III). YabJ is essential for this additional PurR binding, perhaps as a PurR-YabJ complex. The possible association of YabJ with PurR in state III is not addressed.