Two Histone Mutants Enhance the Transcriptional Silencing at rDNA Locus in S. cerevisiae

Histones play an important role in packaging and organizing the genomes. These proteins regulate the accessibility and maintain the stability of the genome. Mutation of histone proteins would lead to alteration of histone core structure and change of transcriptional silencing status. In this study, two histone mutants originated from human cells were characterized in budding yeast. The first histone mutant is known for promoting nucleosome stabilization. By contrast, the other histone mutant is shown to destabilize the nucleosome structure and is the most frequently occurring mutant in cancer cells. The physiological effect of doxycycline induced overexpression of the two histone mutants was observed by using GFP reporters at three major heterochromatin regions: the ribosomal DNA (rDNA), the silent mating, and the telomeric loci. We determined that both histone mutants enhanced the transcriptional silencing at the rDNA locus but not the silent mating and the telomeric loci. Additionally, in order to further elucidate the association between this enhanced silencing effect and posttranslational modification of the histones such as deacetylation and methylation, deletion strains of Sir2, a histone deacetylase, and Set1, a methyltransferase, were constructed. We found that the silencing enhancement of both histone mutants depended on Sir2 but only one of them is Set1-dependent