A role for a bent DNA structure in E2F-mediated transcription activation.
We examined the role of promoter architecture, as well as that of the DNA-bending capacity of the E2F transcription factor family, in the activation of transcription. DNA phasing analysis revealed that a consensus E2F site in the E2F1 promoter possesses an inherent bend with a net magnitude of 40 +/-2 degrees and with an orientation toward the major groove relative to the center of the E2F site. The inherent DNA bend is reversed upon binding of E2F, generating a net bend with a magnitude of 25 +/- 3 degrees oriented toward the minor groove relative to the center of the E2F site. We also found that three members of the E2F family, in conjunction with the DP1 protein, bend the DNA toward the minor groove, suggesting that DNA bending is a characteristic of the entire E2F family. The Rb-E2F complex, on the other hand, does not reverse the intrinsic DNA bend. Analysis of a series of E2F1 deletion mutants defined E2F1 sequences which are not required for DNA binding but are necessary for the DNA-bending capacity of E2F. An internal region of E2F1, previously termed the marked box, which is highly homologous among E2F family members, was particularly important in DNA bending. We also found that a bent DNA structure can be a contributory component in the activation of the E2F1 promoter but is not critical in the repression of that promoter in quiescent cells. This finding suggests that E2F exhibits characteristics typical of modular transcription factors, with independent DNA-binding and transcriptional activation functions, but also has features of architectural factors that alter DNA structure.
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