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Evolution from an all-α to an α + β fold in the Cro protein family. The N-terminal half of the sequence (gray) contains the helix-turn-helix DNA-binding motif and is mostly structurally conserved, while the structurally divergent C-terminal half (orange) contains much of the homodimer interface. Xfaso 1 (3BD1, chain A) and Pfl 6 (2PIJ, chain A), respectively, represent the ancestral and descendant folds and have 40% sequence identity across this 65-residue alignment.
