Atomic force microscopy proposes a 'kiss and pull' mechanism for enhancer function

The DNase I-hyper-sensitive sites (HS2-HS4) in the β-globin gene enhancer region (locus control region; LCR) have been known as the target of Bach1/ MafK heterodimers. We have demonstrated previously by utilizing atomic force microscopy (AFM) that Bach1/MafK mediates the formation of a looped-DNA structure in the LCR fragment. Here we perform further detailed analyses of the loop structure formed between each HSs by AFM, and propose a novel model for the enhancer/protein interaction: the Bach1/ MafK heterodimer preferentially binds to HS2 with highest affinity and to HS3 with lower affinity. However, they assemble to each other to form a stable complex of four heterodimers and mediate a DNA-loop formation. Once the DNA loop is formed between HS2 and HS3, the Bach1/MafK complex at the HS3 side leaves the HS3 and starts to slide along the DNA strand towards HS2 with the other side of the complex fixed at the HS2 region. This 'kiss and pull' model will contribute to understand the function of regulatory proteins at enhancer regions in terms of higher-order structure of DNA, e.g. nucleosomes and chromatin.