Sin mutations of histone H3: Influence on nucleosome core structure and function

Sin mutations in Saccharomyces cerevisiae alleviate transcriptional defects that result from the inactivation of the yeast SWI/SNF complex. We have investigated the structural and functional consequences for the nucleosome of Sin mutations in histone H3. We directly test the hypothesis that mutations in histone H3 leading to a SWI/SNF-independent (Sin) phenotype in yeast lead to nucleosomal destabilization. In certain instances this is shown to be true; however, nucleosomal destabilization does not always occur. Topoisomerase I-mediated relaxation of minichromosomes assembled with either mutant histone H3 or wild-type H3 together with histones H2A, H2B, and H4 indicates that DNA is constrained into nucleosomal structures containing either mutant or wild-type proteins. However, nucleosomes containing particular mutant H3 molecules (R116-H and T118-I) are more accessible to digestion by micrococcal nuclease and do not constrain DNA in a precise rotational position, as revealed by digestion with DNase I. This result establishes that Sin mutations in histone H3 located close to the dyad axis can destabilize histone-DNA contacts at the periphery of the nucleosome core. Other nucleosomes containing a distinct mutant H3 molecule (E105-K) associated with a Sin phenotype show very little change in nucleosome structure and stability compared to wild-type nucleosomes. Both mutant and wild-type nucleosomes continue to restrict the binding of either TATA-binding protein/transcription factor IIA (TFIIA) or the RNA polymerase III transcription machinery. Thus, different Sin mutations in histone H3 alter the stability of histone-DNA interactions to various extents in the nucleosome while maintaining the fundamental architecture of the nucleosome and contributing to a common Sin phenotype.