Conformations of the core nucleosome: effects of ionic strength and high mobility group protein 14 and 17 binding on the fluorescence emission and polarization of dansylated methionine-84 of histone H4.

Chicken histone H4 labeled at Met-84 with the fluor N-[(acetylamino)ethyl]-8-naphthyl-amine-1-sulfonic acid has been incorporated into a nucleosome which has physical characteristics virtually identical with those of native core nucleosomes. The fluorescence emission and polarization properties of the labeled nucleosome were measured as a function of ionic strength and the binding of high mobility group (HMG) proteins 14 and 17. Also, the accessibility of the fluor to the quenching agent acrylamide was determined. It was found that the fluorescence emission changes in the range 0.1-1000 mM NaCl are rather small and indicate that no major unfolding of the octamer structure occurs around Met-84 on H4 at least. Five or perhaps six discrete states were found in that ionic strength range. Each has a different accessibility to the quenching agent. The range of accessibilities varied from 9 X 10(-7) to 32 X 10(-7) mol-1 s-1 for 0.1-1000 mM NaCl, respectively. Polarization measurements showed that there was little change in the rotational relaxation lifetime of the fluor at ionic strengths less than 50 mM NaCl. Above this value, the rotational relaxation lifetimes decreased from 107 to 25 ns at 600 mM NaCl, indicating a moderately increased rotational freedom for the fluor. It is suggested that the histone octamer changes its degree of compaction in the range 0.1-600 mM NaCl but that no major protein unfolding occurs.(ABSTRACT TRUNCATED AT 250 WORDS)