Human SWI/SNF Generates Abundant, Structurally Altered Dinucleosomes on Polynucleosomal Templates

Abstract

Human SWI/SNF (hSWI/SNF) is an evolutionarily conserved ATP-dependent chromatin remodeling complex required for transcriptional regulation and cell cycle control. The regulatory functions of hSWI/SNF are correlated with its ability to create a stable, altered form of chromatin that constrains fewer negative supercoils than normal. Our current studies indicate that this change in supercoiling is due to the conversion of up to one-half of the nucleosomes on polynucleosomal arrays into asymmetric structures, termed “altosomes,” each composed of two histone octamers and bearing an asymmetrically located region of nuclease-accessible DNA. Altosomes can be formed on chromatin containing the abundant mammalian linker histone H1 and have a unique micrococcal nuclease digestion footprint that allows their position and abundance on any DNA sequence to be measured. Over time, altosomes spontaneously revert to structurally normal but improperly positioned nucleosomes, suggesting a novel mechanism for transcriptional attenuation as well as transcriptional memory following hSWI/SNF action.

ACKNOWLEDGMENTS

This work was supported by grants to G.R.S. from The Medical Foundation, the National Cancer Institute (CA088835), and the American Cancer Society (RSG-04-188-01-GMC).

We thank Tony Imbalzano, Bob Kingston, and Aruna Ramachandran for critical comments on the manuscript. We also thank Jeff Hansen for 12 by 208 5S arrays with and without H1, Jerry Workman for the p5SG5E4 template, and the National Cell Culture Center for large-scale HeLa FLAG-Ini1 culture.