Figure 6. Memory switch model.
(A) Two events are triggered independently by exposure to 20E: loci rapidly enter a low-expressing state and at the same time slowly switch from the default state to memory. Memory is characterized by high transcription rate in the presence of hormone, and importantly, cells continue to accumulate loci in the memory state after the hormone is withdrawn. When cells are exposed to 20E a second time, the converted memory loci transit into the high-expressing state, resulting in a more robust second induction. (B). Implication of the memory switch model: upon hormone exposure, loci engage in two separate activities, controlled by independent rate constants – entry into the active state (controlled by kA) and transition into the memory state (controlled by kC). In the memory switch model, normal levels of Nup98 are required for loci to accumulate and remain in the memory state (kC>>k-C), such that depletion of Nup98 abrogates the maintenance of the memory state upon removal of hormone.