Structural evidence for evolution of the beta/alpha barrel scaffold by gene duplication and fusion

The atomic structures of two proteins in the histidine biosynthesis pathway consist of beta/alpha barrels with a twofold repeat pattern. It is Likely that these proteins evolved by twofold gene duplication and gene fusion fro m a common half-barrel ancestor. These ancestral domains are not visible as independent domains in the extant proteins but can be inferred from a comb ination of sequence and structural analysis. The detection of subdomain str uctures may be useful in efforts to search genome sequences for functionall y and structurally related proteins.