Inheritance of histone post-translational modifications

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Multicellular organisms are comprised of different cell types that share the same genome. Cell identity is therefore dictated by the subset of genes that are transcribed in each cell type. Factors that regulate gene expression can be divided into two classes: those that maintain genes in an “ON” state and those that maintain genes in an “OFF” state. The major question in the field of gene regulation is how these proteins maintain transcription patterns in replicating cells, when genome duplication requires unwinding of DNA strands and disruption of associated proteins.******Eukaryotic DNA is packaged with a conserved class of proteins, known as histones. Histones are post-translationally modified by enzymes known as "writers", and different post-translational modifications (PTMs) are associated with gene activation and repression. Histone PTMs hold much promise as heritable markers of gene expression states. Histones displaced during DNA replication are deposited close to their original location on nascent DNA. Genetic studies in yeasts and Drosophila have identified numerous writer enzymes that play roles in inheritance of gene expression states. Despite this evidence however, we have failed to determine whether survival of histone PTMs following genome duplication is a general phenomenon. Instead, many studies have shown that histone PTMs are lost during DNA replication due to the presence of “eraser” enzymes, which remove histone PTMs. Interestingly, eraser enzymes may not be ubiquitously present and data exist demonstrating that the activity of erasers is regulated by the environment. The broad goal of our proposed research is to determine whether histone PTMs are heritable and under what conditions. ******To achieve our goal we will: 1) confirm that active histone PTMs are heritable genome-wide, 2) determine whether alteration of eraser activity by the environment affects inheritance of histone PTMs and 3) test whether histone PTMs regulate re-establishment of transcription patterns following DNA replication. We will use S. cerevisiae as a model organism because histones, writers, and erasers are conserved from yeasts to humans and because the experiments we propose would be extremely difficult and expensive to conduct in mammalian cells. ******Significance: Epigenetics refers to the inheritance of gene expression states without alteration of underlying DNA sequence. The epigenetic regulation of DNA-dependent processes has been intensely studied over the last two decades. However, despite the great interest in epigenetic inheritance, the underlying mechanisms involved remain largely unknown. The research described in this proposal will determine whether histone PTMs can and do mediate epigenetic inheritance and will help prepare seven higher-education students for successful careers in research.