Sda crystal and NMR structures. (a) The Cα traces of the three subunits of the asymmetric unit (A, green; B, light blue; C, magenta) are superposed with each other and with well ordered residues of model A of the NMR structure (red) and Gst-Sda from the Gst-Sda–KinB complex (grey). The side of the molecule which interacts with the histidine kinase is indicated by residue Phe25 from Gst-Sda (grey sticks). (b) Stick representation of Bsu-Sda (chain A, green backbone) superposed with Gst-Sda (grey). Residues which form a hydrophobic surface centred on Phe25 are labelled (Bsu-Sda sequence). (c) Superposition of the interaction surfaces formed between Gst-Sda and Gst-KinB (yellow and orange, respectively), Bsu-Sda chains C and B (magenta and light blue, respectively) and Bsu-Sda chains B and A* (pink and blue, respectively). Residues that form a hydrophobic pocket for Phe25 of Sda are labelled (those in parentheses are the analogous residues from Gst–KinB; orange).