The eukaryotic genome is packaged into chromatin, which is composed of a nucleosome unit containing DNA wrapped around a histone octamer. Chromatin components are subject to various modes of post-translational regulation that have many established roles, including functions in recombination, DNA damage repair, and transcription. Acetylation is a key post-translational modification that is mediated by the opposing enzymatic activities of lysine acetyltransferases (HATs) and deacetylases (HDACs). HATs often exist in large multimeric complexes, such as NuA4 and SAGA, both of which are conserved across species. In humans, the essential catalytic subunit of NuA4, Tip60, along with additional essential subunits such as EPC1/2, are associated with multiple carcinomas. We recently reported that the requirement for ESA1, the S. cerevisiae ortholog of Tip60, could be bypassed by concurrent loss of Rpd3L, an opposing HDAC complex. This bypass is promoted by establishing a relatively balanced cellular acetylation state.
Here, we report on the analysis of the remaining essential components of NuA4, including Arp4, Act1, Swc4, Tra1, and Epl1. The bypass state is characterized by a reduction of cellular fitness, including sensitivity to high temperature and DNA damage, low histone H4 acetylation, and cell cycle defects. These phenotypes were selectively suppressed by loss of additional deacetylases such as HDA1. Whereas Esa1 and several other NuA4 subunits are well-described, there are subunits that remain to be fully characterized. Specifically, Epl1, the ortholog of human EPC1/2, although essential, is poorly understood in vivo across species, and is a focal point of interest in this study. Taking advantage of the bypass suppression, we present new functional studies of Epl1 in S. cerevisiae with the aim of better understanding the essential role of this crucial NuA4 subunit. Due to the conservation of Epl1 between budding yeast and humans, many of our basic findings in S. cerevisiae can be translated to human cellular function and biology.