MECHANISMS OF STABILIZING NUCLEOSOME STRUCTURE - STUDY OF DISSOCIATION OF HISTONE OCTAMER FROM DNA

The influence of ionic strength on DNA-histone and histone-histone int eractions in reconstituted nucleosomes was studied by measuring the pa rameters of histone tyrosine fluorescence: fluorescence intensity and lambda(max) position. The first parameter is sensitive to histone-DNA interactions. The changes of the second one accrue due to hydrogen bon d formation/disruption between tyrosines in the histone H2A-H2B dimer and the (H3-H4)(2) tetramer. The simultaneous measurement of these par ameters permits the recording of both the dissociation of histone comp lexes from DNA, as well as changes in histone-histone interactions. As ionic strength is increased, the H2A-H2B histone dimer dissociated fi rst, followed by dissociation of the (H3-H4)(2) tetramer [Yager, T.G., McMurray, C.T. and Van Holde, K.E. (1989) Biochemistry 28, 2271-2276] . The H2A-H2B dimer is dissociated in two stages: first, the ionic bon ds with DNA were disrupted, followed by the dissociation of the histon e dimer from the tetramer. And secondly, the disruption of dimer-tetra mer specific H-bonds. It was established that the energy of electrosta tic interactions of the histone dimer with DNA within the nucleosome i s much less than the energy of interaction of the histone dimer with t he tetramer.