Barrels in pieces?

Imidazole glycerol phosphate synthase (HisF) has been proposed to be the pr oduct of duplication of a gene encoding a (beta/alpha)(4)-half barrel follo wed by fusion to encode the complete (beta/alpha)(8)-barrel. In support of this evolutionary scenario, the N- and C-terminal (beta/alpha)(4)-half barr els of TrpF have been separately expressed and purified. Each assumes a sta ble, soluble homodimeric structure, but neither is catalytically active; wh en expressed together, a functional heterodimer is formed.