Abstract 1065 Determining the Effects of Poly-A Tracts and Monovalent Cations on Nucleosome Equilibrium and Dynamics

Nucleosomes, composed of 147 base pairs of DNA wrapped around an octamer of histone proteins, play a role in gene regulation and cellular function by controlling access to the genetic information stored within DNA. The placement of nucleosomes along strands of DNA is determined, in part, by the presence of homopolymeric stretches of deoxyadenosine nucleotides on one strand of double stranded DNA, commonly referred to as poly-A tracts, which are overabundant in eukaryotic genomes. These poly-A tracts exclude nucleosomes in a length and purity dependent manner. We have previously shown that A-tracts adopt unique structures that are preferentially stabilized by specific monovalent cations. The exclusion of nucleosomes by these long A-tracts, greater than 15 base pairs, impacts the accessibility of nearby DNA. The equilibrium accessibility of DNA within the nucleosomes, determined by unwrapping and rewrapping rates, is also an important factor in regulating DNA dependent processes. The impact of short A-tracts, 6-8 base pairs, on nucleosome dynamics is poorly understood. Using competitive reconstitution in the presence of different monovalent cations, we have investigated how the length and placement of short A-tracts within the nucleosome affects the ability of nucleosomes to form. We demonstrate that the same cations that stabilize the unique A-tract structure exacerbate the effects of A-tracts on nucleosome stability. We further demonstrate that the effect depends on the location of the A-tract within the nucleosome. Our hypothesis is that the A-tracts will also impact the equilibrium accessibility by slowing down the rewrapping rate in a metal ion dependent manner. Using equilibrium and stopped-flow FRET measurements we are studying how the A-tract and monovalent cation dependent changes in nucleosome stability impact nucleosome dynamics. This work was funded and supported by the University of Wisconsin - La Crosse Dean's Distinguished Fellowships from Jes Rasmussen and Colleen Cooper, and the Joe and Suzanne Toce, and by University of Wisconsin - La Crosse Undergraduate Research and Creativity Grants.