The B-WICH chromatin-remodelling complex regulates RNA polymerase III transcription by promoting Max-dependent c-Myc binding.

The chromatin-remodelling complex B-WICH, comprised of William syndrome transcription factor, the ATPase SNF2h and nuclear myosin, specifically activates RNA polymerase III transcription of the 5S rRNA and 7SL genes. However, the underlying mechanism is unknown. Using high-resolution MN walking we demonstrate here that B-WICH changes the chromatin structure in the vicinity of the 5S rRNA and 7SL RNA genes during RNA polymerase III transcription. The action of B-WICH is required for the binding of the RNA polymerase machinery and the regulatory factors c-Myc at the 5S rRNA and 7SL RNA genes. In addition to the c-Myc binding site at the 5S genes, we have revealed a novel c-Myc and Max binding site in the intergenic spacer of the 5S rDNA. This region also contains a region remodelled by B-WICH. We demonstrate that c-Myc binds to both sites in a Max-dependent way, and thereby activate transcription by acetylating histone H3. The novel binding patterns of c-Myc and Max link transcription of 5S rRNA to the Myc/Max/Mxd network. Since B-WICH acts prior to c-Myc and other factors, we propose a model in which the B-WICH complex is required to maintain an open chromatin structure at these RNA polymerase III genes. This is a prerequisite for the binding of additional regulatory factors.