NMR STRUCTURE AND COMPARISON OF THE ARCHAEAL HISTONE HFOB FROM THE MESOPHILE METHANOBACTERIUM-FORMICICUM WITH HMFB FROM THE HYPERTHERMOPHILE METHANOTHERMUS-FERVIDUS

The solution-state structure of the recombinant archaeal histone rHFoB , from the mesophile Methanobacterium formicicum, has been determined by two- and three-dimensional (3D) proton homonuclear correlated nucle ar magnetic resonance (NMR) methods. On the basis of 951 nuclear Overh auser effect (NOE)-derived distance restraints, rHFoB monomers form th e histone fold and assemble into symmetric (rHFoB)(2) dimers that have a structure consistent with assembly into archaeal nucleosomes. rHFoB exhibits similar to 78% sequence homology with rHMfB from the hyperth ermophile Methanothermus fervidus, and the results obtained demonstrat e that these two proteins have very similar 3D structures, with a root -mean-square deviation for backbone atoms of 0.65 +/- 0.13 Angstrom(2) . (rHFoB)(2) dimers however unfold at lower temperatures and require a higher salt environment for stability than (rHMfB)(2) dimers, and com paring the structures, we predict that these differences result from u nfavorable surface-located ionic interactions and a larger, more solve nt-accessible cavity adjacent to residue G36 in the hydrophobic core o f (rHFoB)(2).