SOLUTION STRUCTURE OF THE HU PROTEIN FROM BACILLUS-STEAROTHERMOPHILUS

The histone-like protein HU from Bacillus stearothermophilus is a dimer with a molecular mass of 19.5 kDa that is capable of bending DNA. An X-ray structure has been determined, but no structure could be established for a large part of the supposed DNA-binding beta-arms. Using distance and dihedral constraints derived from triple-resonance NMR data of a C-13/N-15 doubly-labelled HU protein 49 distance geometry structures were calculated, which were refined by means of restrained Molecular Dynamics, From this set a total of 25 refined structures were selected having low constraint energy and few constraint violations. The ensemble of 25 structures display a root-mean-square co-ordinate deviation of 0.36 Angstrom with respect to the average structure, calculated over the backbone heavy atoms of residues 2 to 54 and 75 to 90 (and residues 2' to 54' and 75' to 90' of the second monomer). The structure of the core is very similar to that observed in the X-ray structure, with a pairwise r.m.s.d. of 1.06 Angstrom. The structure of the beta-hairpin arm contains a double flip-over at the prolines in, the two strands of the beta-arm. Strong N-15-NH heteronuclear nuclear Overhauser effects indicate that the beta-arm and especially the tip is flexible. This explains the disorder observed in the solution and X-ray structures of the beta-arm, in respect of the core of the protein. Overlayed onto itself the beta-arm is better defined, with an r.m.s.d. of 1.0 Angstrom calculated over the backbone heavy atoms of residues 54 to 59 and 69 to 74. The tip of the arm adopts a well-defined 4:6 beta-hairpin conformation similar to the iron co-ordinating beta-arms of rubredoxin. (C) 1995 Academic Press Limited