within about 15 minutes to two hours. The double-strand breaks in the promoter are thus associated with TOP2B and at least these four repair enzymes. These proteins are present simultaneously on a single promoter nucleosome (there are about 147 nucleotides in the DNA sequence wrapped around a single nucleosome) located near the transcription start site of their target gene. The double-strand break introduced by TOP2B apparently frees a part of the promoter at an RNA polymerase-bound transcription start site to physically move to its associated enhancer (see regulatory sequence). This allows the enhancer, with its bound transcription factors and mediator proteins, to directly interact with the RNA polymerase paused at the transcription start site to start transcription. Contextual fear conditioning in the mouse causes the mouse to have a long-term memory and fear of the location in which it occurred. This conditioning causes hundreds of gene-associated DSBs in the medial prefrontal cortex (mPFC) and hippocampus that are important for learning and memory. Other TOP2B and DNA interactions Interactions of TOP2B and DNA also regulate transcription of genes that are important for development. These include genes encoding axon guidance factors and cell adhesion molecules. Specifically, TOP2B is required for lamina-specific targeting of retinal ganglion cell axons and dendrites in the zebrafish. Interactions TOP2B has been shown to interact with: BAZ1B, HDAC1, Histone deacetylase 2, P53, Small ubiquitin-related modifier 1 and T-cell surface glycoprotein CD3 epsilon chain. In other species In Drosophila Hadlaczky et al 1988 found DNA topoisomerase II β did not correlate with cell proliferation - while α did. References Further reading