Abstract: The epigenome influences gene expression and chromosome dynamics without causing any changes in their DNA sequence. Heavy hitters in the world of epigenetics include the family of histone proteins, such as the nucleosome forming H2A, H2B, H3, and H4 proteins. Other histones include H1, present on linker DNA outside of the core histone octamer, and H2A.Z, a variant of H2A present in the nucleosome. Studies have implicated the histone proteins in gene expression, chromatin structure, and chromatin stability, but many more functions remain undiscovered. In this study, we investigate the influence of histones H1 and H2A.Z in DNA repair via Non-Homologous End Joining (NHEJ), and chromosome stability measured by plasmid retention rates. To study NHEJ efficacy, we transformed Δleu2 yeast strains (S. cerevisiae) lacking histone H1, H2A.Z, or both with a plasmid containing the LEU2 gene (linearized by endonuclease HindIII) necessary for their de novo leucine synthesis. As this plasmid lacks homology with the yeast genome, transformants must repair the cut plasmid (via NHEJ) to propagate the plasmid and allow colony growth of each strain on media lacking leucine. When studying chromosome stability, we used the same Δleu2 strains transformed with the LEU2 plasmid and plated them onto non-selective media (YPD). We then replica plated/transferred the colonies on YPD to media lacking leucine (SDC-Leu), selecting for cells that still retained the LEU2 plasmid. This way, we quantify each strain’s plasmid retention rates by comparing colony growth on selective (SDC-Leu) and non-selective media (YPD). Through these approaches, our lab found NHEJ and plasmid retention defects in yeast lacking H2A.Z or both H1 and H2A.Z. These findings directly implicate histone H2A.Z in DNA repair and chromosome stability, uncovering a bit more about the complex genetic landscape called the epigenome.
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