3-DIMENSIONAL STRUCTURE OF THE DNA-BINDING DOMAIN OF THE FRUCTOSE REPRESSOR FROM ESCHERICHIA-COLI BY H-1 AND N-15 NMR

FruR is an Escherichia coli transcriptional regulator that belongs to the LacI DNA-binding protein family. By using H-1 and N-15 NMR spectro scopy, we have determined the three-dimensional solution structure of the FruR N-terminal DNA-binding domain consisting of 57 amino acid res idues. A total of 809 NMR-derived distances and 54 dihedral angle cons traints have been used for molecular modelling with the X-PLOR program . The resulting set of calculated structures presents an average root- mean-square deviation of 0.37 Angstrom at the main-chain level for the first 47 residues. This highly defined N-terminal part of the structu re reveals a similar topology for the three a-helices when compared to the 3D structures of LacI and PurR counterparts. The most striking di fference lies in the connection between helix II and helix III, in whi ch three additional residues are present in FruR. This connecting segm ent is well structured and contains a type III turn. Apart from hydrop hobic interactions of non-polar residues with the core of the domain, this connecting segment is stabilised by several hydrogen bonds and by the aromatic ring stacking between Tyr19 of helix II and Tyr28 of the turn. The region containing the putative ''hinge helix'' (helix TV), that has been described in PurR-DNA complex to make specific base cont acts in the minor groove of DNA, is unfolded. Examination of hydrogen bonds highlights the importance of homologous residues that seem to be conserved for their ability to fulfil helix N and C-capping roles in the LacI repressor family. (C) 1997 Academic Press Limited.