FIGURE 4.

Secondary structure of FixK₂. The secondary structure elements (determined with the software program DSSP using the structure of FixK₂(C183S)-His₆) are colored red (α-helices) and blue (β-strands). Amino acid residues and secondary structure elements (predicted from the sequence using PSIPred) that are not seen in the solved structure due to the lack of electron density are depicted in lighter colors. Additional structural elements relevant for this work are highlighted as follows: box, HTH motif; white letters, amino acids in the LEXXXR motif of helix αF making contact with DNA at the FixK₂ box; asterisk, oxidation-sensitive Cys-183; and underlining, the 12 amino acids that are cleaved off when the C terminus is not protected by a His₆ tag.