The structural information of extending ketosynthase. (A) Crystal structure of FabF. (PDB: 2GFW) (B) Crystal structure of actKS-CLF with KS colored in gray and CLF colored in green. (PDB: 1TQY) The substrate pocket (yellow) extends from the KS subunit into the CLF subunit. Another pocket with unknown purpose is found in the CLF subunit (blue). (C) Four gates in KS-CLF that determine the chain length, using actKS-CLF crystal structure as an illustration (KS in gray and CLF in green). The bulky gatekeeper residues are colored red in the protein sequence of different KS-CLFs. Act = actinorhodin, Tcm = tetracenomycin, Whi = WhiE spore pigment, and Gri = griseorhodin. The parenthesis after each KS-CLF indicates the chain length of the respective polyketide product. The substrate extends from the right to the left of the figure, thus, the earlier the gate is blocked, the shorter the product is.