A model of the Tsr dimer. The two subunits are distinguished by the different shading of the α-helices, which are represented by rectangles that are proportional to the helix lengths. N and C indicate the positions of the N- and C-termini, respectively. The approximate locations of the serine binding site (◇), the CheW binding site (W), and the methylation sites are displayed. The linker region (~ residues 215–265) is represented by a dashed segment, which is not to scale, simply to indicate continuity in the primary structure. The four major sites (E297, E304, E311, and E493) are depicted as filled circles and are labeled 1 to 4. One minor site (E502) is depicted as an open circle. The first three sites are located sequentially on MH1; the fourth (and fifth) site(s) are located on MH2, which is antiparallel to MH1.