Long range interaction of cis-DNA elements mediated by architectural transcription factor Bach1.

Abstract

A central question in vertebrate transcriptional regulation is how cis-regulatory modules, including enhancers, silencers and promoters, communicate with each other over long distances to mandate proper gene expression. In order to address this question we analysed protein/DNA interactions in the human beta-globin locus control region (LCR). One of the many proteins that are potentially implicated in LCR function is Bach1. Bach1 possesses a basic leucine zipper (bZip) domain, as well as a BTB/POZ domain that has been shown to be involved in the regulation of chromatin structure. Bach1 forms heterodimers with small Maf proteins through its leucine zipper and binds to Maf recognition elements (MARE). Using atomic force microscopy we visualized large looped DNA structures between MAREs located in different regulatory elements within the human beta-globin LCR that were mediated by Bach1/MafK heterodimers. The formation of these DNA loops required the Bach1 BTB/POZ protein interaction domain. Furthermore, in transfection studies we found that Bach1 repressed the enhancer activity of the LCR in a BTB/POZ domain-dependent manner. Our results suggest that Bach1 and other BTB/POZ transcription factors may represent a class of nuclear architectural proteins that mediate long range interactions between cis-regulatory elements in order to regulate gene expression.