A hyper-dynamic equilibrium between promoter-bound and nucleoplasmic dimers controls NF-κB-dependent gene activity

Because of its very high affinity for DNA, NF- kappa B is believed to make long- lasting contacts with cognate sites and to be essential for the nucleation of very stable enhanceosomes. However, the kinetic properties of NF- kappa B interaction with cognate sites in vivo are unknown. Here, we show that in living cells NF- kappa B is immobilized onto high- affinity binding sites only transiently, and that complete NF- kappa B turnover on active chromatin occurs in less than 30 s. Therefore, promoter- bound NF- kappa B is in dynamic equilibrium with nucleoplasmic dimers; promoter occupancy and transcriptional activity oscillate synchronously with nucleoplasmic NF- kappa B and independently of promoter occupancy by other sequence- specific transcription factors. These data indicate that changes in the nuclear concentration of NF- kappa B directly impact on promoter function and that promoters sample nucleoplasmic levels of NF- kappa B over a timescale of seconds, thus rapidly re- tuning their activity. We propose a revision of the enhanceosome concept in this dynamic framework.