Low-level p53 expression changes transactivation rules and reveals superactivating sequences.

Transcriptional activation by the tumor suppressor p53 is considered to depend on cellular level, although there are few systems where this dependence on cellular level of p53 has been directly addressed. Previously, we reported that transactivation from p53 targets was sensitive to both p53 amount and DNA sequence, with some sequences being responsive to much lower p53 levels than others when examined in yeast model systems or human cells. Because p53 is normally present at low levels and perturbations might lead to small increases, we examined transactivation under limiting p53. Unlike the positive relationship between transactivation and binding affinity from target sequences at high cellular levels of human p53 in yeast, no such relationship was found at low levels. However, transactivation in the yeast system and the torsional flexibility of target sequences were highly correlated, revealing a unique structural relationship between transcriptional function and sequence. Surprisingly, a few sequences supported high transactivation at low p53 levels in yeast or when transfected into human cells. On the basis of kinetic and flexibility analyses the "supertransactivation" property was due to low binding off rates of flexible target sites. Interestingly, a supertransactivation response element can differentiate transcriptional capacities of many breast cancer-associated p53 mutants. Overall, these studies, which are relevant to other transcription factors, address the extent to which transactivation properties of p53 target sequences are determined by their intrinsic physical properties and reveal unique rules of engagement of target sequences at low p53 levels.