HMG I/Y regulates long-range enhancer-dependent transcription on DNA and chromatin by changes in DNA topology

The nature of nuclear structures that are required to confer transcriptiona l regulation by distal enhancers is unknown. We show that long-range enhanc er-dependent beta-globin transcription is achieved in vitro upon addition o f the DNA architectural protein HMG I/Y to affinity-enriched hole RNA polym erase II complexes. In this system, HMG I/Y represses promoter activity in the absence of an associated enhancer and strongly activates transcription in the presence of a distal enhancer. Importantly, nucleosome formation is neither necessary for long-range enhancer regulation in vitro nor sufficien t without the addition of HMG I/Y, Thus, the modulation of DNA structure by HMG I/Y is a critical regulator of long-range enhancer function on both DN A and chromatin-assembled genes, Electron microscopic analysis reveals that HMG I/Y binds cooperatively to preferred DNA sites to generate distinct lo oped structures in the presence or absence of the beta-globin enhancer, The formation of DNA topologies that enable distal enhancers to strongly regul ate gene expression is an Intrinsic property of HMG I/Y and naked DNA.