Reconstructing the Remote Origins of a Fold Singleton from a Flavodoxin-Like Ancestor

[Image: see text] Evolutionary processes that led to the emergence of structured protein domains left footprints in the sequences of modern proteins. We searched for such hints employing state-of-the-art sequence analysis and found evidence that the HemD-like fold emerged from the flavodoxin-like fold through segment swap and gene duplication. To verify this hypothesis, we reverted these evolutionary steps experimentally, constructing a HemD-half that resulted in a protein with the canonical flavodoxin-like architecture. These results of fold reconstruction from the sequence of a different fold strongly support our hypothesis of common ancestry. It further illustrates the plasticity of modern proteins to form new folded proteins.