The effect of age on HO expression in yeast cells

Open Access
Paskoff, Benjamin Lewis
Area of Honors:
Biochemistry and Molecular Biology
Bachelor of Science
Document Type:
Thesis Supervisors:
  • Lu Bai, Thesis Supervisor
  • Sarah Ades, Honors Advisor
  • Yeast
  • Aging
  • HO Promoter
  • Nucleosome Occupancy
  • Transcription factor binding
Histones are an important component of chromatin structure that affects gene regulation. DNA in our cells is wound around an octamer of four core histone proteins that make up a nucleosome. Nucleosome occupancy plays a large role in regulating transcription by controlling the accessibility of DNA for transcription factors to bind and signal promoters downstream. Recent studies have linked lower histone concentration to aging and have even increased cell lifespan by overexpressing histones (Feser et al., 2010). Consequently, it was proposed that as cells lose histone concentration, nucleosome occupancy may be affected as well. This could create inappropriate cryptic transcription of the DNA when transcription factors are suddenly able to bind to the newly formed nucleosome depleted regions (Berretta and Morillon, 2009). Here we tested this hypothesis by analyzing HO expression in aged yeast cells. We chose the HO promoter because it is one of the best-characterized promoters in yeast and its activation is highly sensitive to nucleosome occupancy. If nucleosome occupancy is decreased in aged cells, we expect to see an increase in HO activation resulting from increased accessibility along the promoter for transcription factor binding. Our results showed that HO expression decreases as the cells age contradicting our original hypothesis. This result indicates that nucleosome loss does not appear to be occurring in the HO promoter. In addition, there appears to be a transcription activator or co-activator critical to driving HO expression that is losing efficacy in aged cells. Mating factors were tested for potential repression of HO, and SBF’s binding capacity in older cells was evaluated in CLN2 and HO-Reb1*, however, neither appeared to be responsible for decreasing expression of HO.