Using the decay of intrinsic tyrosine fluorescence of core particles to monitor conformational changes

摘要

We have examined the effect of ionic strength on the decay of intrinsic tyrosine fluorescence from chromatin core particles. All measured decays were complex, showing at least four components. At 0.1 M salt the average lifetime was ～0.5 ns with 67% of the emitting tyrosines having lifetimes on the order of 0.3 ns, 30% near 0.8 ns and the remainder around 1.8 ns. The average lifetime increases continuously by about 50% as the ionic strength is decreased to very low values. The decrease was characterized by a small shift in contributions from components with ～0.3 ns lifetimes mainly to components with lifetimes on the order of 1.8 ns. The rate of change was greatest over the range of the low-salt transition ([Na] < 0.5 mM). As the salt concentration was increased beyond 0.1 M a large increase in average lifetime is observed as the histone proteins within the core particle dissociate from the DNA. The average lifetime increased to a maximum of ～1.6 ns due to a large decrease in contribution from components with ～0.3 ns lifetimes accompanied by the appearance of components with lifetimes mainly from ～1.8 to ～4.0 ns. Throughout the range of salt examined the changes in average decay lifetime paralleled changes in the steady-state fluorescence intensity. This agreement indicates that all tyrosines contributing to the fluorescence at very high and very low salt also contribute at intermediate (0.1 M) salt. It is not necessary, as suggested by others, to consider some tyrosines to be statically quenched in the intact core particle.