CFRFSS · Chromatin Fiber and Remodeling Factor Structural Studies

DNA in higher organisms is organized in a nucleoprotein complex called chromatin. The structure of chromatin is responsible for compacting DNA to fit within the nucleus and for governing its access in nuclear processes. Epigenetic information is encoded chiefly via chromatin modifications. Readout of the genetic code depends on chromatin remodeling, a process actively altering chromatin structure. An understanding of the hierarchical structure of chromatin and of structurally based, remodeling mechanisms will have enormous impact for developments in medicine.Following our high resolution structure of the nucleosome core particle, the fundamental repeating unit of chromatin, we have endeavored to determine the structure of the chromatin fiber. We showed with our X-ray structure of a tetranucleosome how nucleosomes could be organized in the fiber. Further progress has been limited by structural polymorphism and crystal disorder, but new evidence on the in vivo spacing of nucleosomes in chromatin should stimulate more advances. Part A of this application describes how we would apply these new findings to our cryo-electron microscopy study of the chromatin fiber and to our crystallographic study of a tetranucleosome containing linker histone.Recently, my laboratory succeeded in providing the first structurally based mechanism for nucleosome spacing by a chromatin remodeling factor. We combined the X-ray structure of ISW1a(ATPase) bound to DNA with cryo-EM structures of the factor bound to two different nucleosomes to build a model showing how this remodeler uses a dinucleosome, not a mononucleosome, as its substrate. Our results from a functional assay using ISW1a further justified this model. Part B of this application describes how we would proceed to the relevant cryo-EM and X-ray structures incorporating dinucleosomes. Our recombinant ISW1a allows us to study in addition the interaction of the ATPase domain with nucleosome substrates.