Excess histone levels mediate cytotoxicity via multiple mechanisms

The accumulation of excess histone proteins in cells has deleterious consequences such as genomic instability in the form of excessive chromosome loss, enhanced sensitivity to DNA damaging agents and cytotoxicity. Hence, the synthesis of histone proteins is tightly regulated at multiple steps and transcriptional as well as posttranscriptional regulation of histone proteins is well established. Additionally, we have recently demonstrated that histone protein levels are regulated posttranslationally by the DNA damage checkpoint kinase Rad53 and ubiquitin-proteasome dependent proteolysis in the budding yeast. However, the underlying mechanism/s via which excess histones exert their deleterious effects in vivo are not clear. Here we have investigated the mechanistic basis for the deleterious effects of excess histones in budding yeast. We find that the presence of excess histones saturates certain histone modifying enzymes, potentially interfering with their activities. Additionally, excess histones appear to bind non-specifically to DNA as well as RNA, which can adversely affect their metabolism. Microarray analysis revealed that upon overexpression of histone gene pairs, about 240 genes were either up or downregulated by 2-fold or more. Overall, we present evidence that excess histones are likely to mediate their cytotoxic effects via multiple mechanisms that are primarily dependent on inappropriate electrostatic interactions between the positively charged histones and diverse negatively charged molecules in the cell. Our findings help explain the basis for the existence of multiple distinct mechanisms that contribute to the tight control of histone protein levels in cells and highlight their importance in maintaining genomic stability and cell viability.