H-1, C-13, AND N-15 RESONANCE ASSIGNMENTS AND SECONDARY STRUCTURE-ANALYSIS OF THE HU PROTEIN FROM BACILLUS-STEAROTHERMOPHILUS USING 2-DIMENSIONAL AND 3-DIMENSIONAL DOUBLE-RESONANCE AND TRIPLE-RESONANCE HETERONUCLEAR MAGNETIC-RESONANCE SPECTROSCOPY

Nearly complete H-1,C-13, and N-15 resonance assignments have been obtained for the protein HU from Bacillus stearothermophilus (dimer, 19.5 kDa) using double- and triple-resonance 2D and 3D NMR experiments. This has resulted in assignments of 91% of the observable protons, 98% of all C-13, and 92% of all N-15 nuclei. NOEs obtained from a 3D time-shared NOESY-(C-13,N-15)-HSQC spectrum, exchange data of amide protons, and chemical shifts of the H-1(alpha), H-1(N), C-13(beta), C-13(alpha), (CO)-C-13 and N-15 nuclei have been used to identify the secondary structure elements. Three alpha-helices (residues 3-13, 18-37, and 83-90) and three extended strands (residues 40-45, 48-62, and 67-82) have been found in HU. The arrangement of these elements of secondary structure is very similar to the X-ray structure [Tanaka et al. (1984) Nature 310, 376-381; White et al. (1989) Proteins 5, 281-288]. The conformation of the proposed DNA-binding region of HU, i.e., an antiparallel beta-hairpin, was not observed previously in the X-ray structure. In the NMR structure long range NOEs in the beta-arm region (residues 53-76) suggest a distortion between residue Pro-72 and Ala-73 and between Pro-63 and Gln-64 with concomitant distortions in the opposite strand. The NOE data indicate further that the loop region in the DNA-binding arms of HU is arranged as a type I beta-turn from Pro-63 to Gly-66.