The SIN domain of the histone octamer is essential for intramolecular folding of nucleosomal arrays.

The SIN domain within histones H3 and H4 is defined by a set of single amino acid substitutions that were initially identified as mutations that alleviate the transcriptional defects associated with inactivation of the SWI/SNF chromatin remodeling complex. Here we use recombinant histones to investigate how Sin- versions of H4 alter the structure of nucleosomal arrays. We find that an R45C substitution within the SIN domain of H4 does not disrupt nucleosome positioning nor does this Sin- version alter the accessibility of nucleosomal DNA. In contrast, we find that the R45C substitution eliminates Mg2+-dependent, intramolecular folding of the nucleosomal arrays. Our results suggest that Sin- versions of histones may alleviate the need for SWI/SNF in vivo by disrupting higher-order chromatin folding.