Cre exists in two distinct conformations with one pair of subunits in conformation 1, shown in green, and the other pair in conformation 2, shown in purple (Fig. 12-9b).Only in one of these conformations (the green subunits in the figure) can Cre cleave and rejoin DNA.Thus, only one pair of subunits is in the active conformation at a time.The pair of subunits in this active conformation switches as the reaction progresses. This switching is critical for controlling the progress of recombination and ensuring the sequential “one strand at a time”exchange mechanism.