(A) Ribbon diagram of the MtmB hexamer. The subunits forming the two trimers are shaded in red and blue hues, respectively. (B) Ribbon diagram of one MtmB subunit colored by secondary structure element (a helices, red; b sheets, cyan; random coil, green). The atoms of the UAGencoded residue are shown as ball-and-stick models and are colored by their elements, with carbon as gray, nitrogen as blue, and oxygen as red. These .gures were prepared with the programs MOLSCRIPT, and Raster3D (22, 23).