3-DIMENSIONAL DIMER STRUCTURE OF THE LAMBDA-CRO REPRESSOR IN SOLUTIONAS DETERMINED BY HETERONUCLEAR MULTIDIMENSIONAL NMR

The H-1, N-15 and C-13 magnetic resonances of the lambda-Cro repressor have been assigned almost completely mainly through the use of hetero nuclear multidimensional NMR methods. Inter-subunit NOEs were distingu ished by means of heteronuclear spectral editing technique (C-13 doubl e half filter technique). Based on the distance and dihedral angle con straints derived from the NMR data, the three-dimensional solution str ucture of the lambda-Cro repressor in the dimeric form has been calcul ated by the simulated annealing method. The input for the structure ca lculations consisted of H-1-H-1 distance constraints, of which 1536 we re intra-subunit and 40 were inter-subunit, and dihedral angle, phi, c onstraints, which numbered 92. The average root-mean-square deviation (RMSD) for all backbone heavy-atoms of the 20 calculated structures fo r residues 3 to 59 of the total of 66 amino acid residues in both subu nits was 1.57 Angstrom, while the average RMSD for each subunit in the same residue range was 0.66 Angstrom. The subunit is composed of thre e alpha-helices, residues 7 to 13, 16 to 23 and 27 to 36, and a three- stranded anti-parallel beta-sheet composed of residues 3 to 6, 40 to 4 4 and 50 to 55. The solution structure of the subunit is essentially t he same as that in the crystalline form, but the structure of the dime r form in solution differs from that of the dimer unit in the crystall ine form. It is suggested that the solution dimer structure is distort ed to fit the recognition helices in the major grooves of DNA on compl ex formation. (C) 1995 Academic Press Limited