Fig. 4.

Evolution of the recursive domain of desulfoferrodoxin (1dfx, residues 37–125). In each panel, strands that are operands for a folding rule are shown in arbitrarily chosen colors, and remaining parts of the structure are in white. The colors are reassigned in each successive panel. Initially, three hairpins are identified and colored red, green, and chartreuse in a. On application of the antiparallel bridge rule, the blue, chartreuse, red, and green strands in b resolve into a single recursive domain, shown in chartreuse in c. Then, on application of the indirect β-wind rule to the chartreuse, blue, red, and green strands in d, the entire fold reduces to a single recursive domain, shown in chartreuse (e).