Use of the transglutaminase reaction to study the dissociation of histone N-terminal tails from DNA in nucleosome core particles.

We have recently shown that core histones are glutaminyl substrates for transglutaminase (TGase) and that when native nucleosome cores are incubated with monodansylcadaverine (DNC) as donor amine, this fluorescent probe is incorporated into Gln5 and Gln19 of H3 and in Gln22 of H2B [Ballestar et al. (1996) J. Biol. Chem. 271, 18817-18825]. In the present paper, we report that the cause by which Gln22 of H2B is modified in nucleosomes but not in the free histone is the interaction of the region containing that glutamine with DNA. We have used the specificity of the TGase reaction to study the changes induced by increasing ionic strength in the interaction between the histone N-terminal tails and nucleosome DNA by two different approaches. First, the reactivity of the histone tail glutamines was employed to monitor changes in the interactions between the regions containing these residues and DNA. Second, by using reconstituted nucleosome core particles containing either H2B modified with DNC by the TGase reaction at Gln22 or H3 modified with the same procedure at Gln5 and Gln19, the dissociation of the histone tails was followed by the decrease of the fluorescence anisotropy of the probe. These methods allowed us to describe two ionic strength dependent structural transitions of the histone tails not reported to date. In the case of H2B, the first one occurs at very low ionic strength, and it can be assigned to an increase in the mobility of Gln22. The second one results in the cooperative release of a region of the tail that includes lysine residues next to Gln22, and it is followed by the overall release of the entire tail, described by other workers.