Nucleosome dynamics. VI. Histone tail regulation of tetrasome chiral transition. A relaxation study of tetrasomes on DNA minicircles

We have recently described the relaxation of mononucleosomes on an homologo us series of 351-366 bp DNA minicircles, as a tool to study nucleosome stru cture and dynamics in vitro. Nucleosomes were found to have a tail-regulate d access to three distinct DNA conformations, depending on the crossing bet ween the entering and exiting DNAs, and its polarity. This approach was now used to explore tetrasome chiral transition, and the influence of the hist one tails. The data confirmed the existence of two states, with linking num ber differences Delta Lk(t) = -0.74(+/-0.01) and +0.51(+/-0.06). As expecte d, the particle free energy is higher in the right-handed state (Delta G(t) =1.9(+/-0.1) kT), but it decreased (to 1.3(+/-0.1) kT) upon histone acetyla tion and the addition of phosphate, a potent tail destabilizer. Removal of the tails with trypsin further decreased Delta G(t) (to 0.6 kT), and also i nduced a loss of supercoiling in both states, to Delta Lk(t) = -0.64(+/-0.0 3) and +0.35(+/-0.05). The loop end-conditions, and hence the parameters of the DNA superhelix, were then calculated for both states using the explici t solutions to the equations of the mechanical equilibrium in the theory of elastic rod model for DNA. Whereas the pitch of the DNA superhelix may be approximately equal and opposite in the two conformations, its radius (r) w as 20% larger in the right-handed conformation, confirming previous observa tions by electron microscopy of a tetrasome lateral opening in that conform ation. The above supercoiling losses were found to reflect a further 3% inc rease in r (to 23%) upon removal of the tails in the right-handed conformat ion, and a 14% increase in the left-handed conformation. The use of composi te tetramers with one histone tail intact and the other removed showed thes e effects to be essentially due to the H3 tails. Altogether, these results show that the H3 tails oppose the tetrasome opening which is expected to be required to relieve the clash between the entering and exiting DNAs in the course of the transition, but which also appears to be intrinsic to the pr otein reorientation mechanism. We propose that the block against opening re sults from the H3 tails intercalating into the small groove of the double h elix at +/-10 bp from the dyad, and acting as wedges against local DNA stra ightening. The tails (especially H3) may therefore regulate tetrasome chira l transition in vivo. (C) 2000 Academic Press.