The Ketosynthase Domain Controls Chain Length in Mushroom Oligocyclic Polyketide Synthases

The nonreducing iterative type I polyketide synthases (NR‐PKSs) CoPKS1 and CoPKS4 of the webcap mushroom Cortinarius odorifer share 88 % identical amino acids. CoPKS1 almost exclusively produces a tricyclic octaketide product, atrochrysone carboxylic acid, whereas CoPKS4 shows simultaneous hepta‐ and octaketide synthase activity and also produces the bicyclic heptaketide 6‐hydroxymusizin. To identify the region(s) controlling chain length, four chimeric enzyme variants were constructed and assayed for activity in Aspergillus niger as heterologous expression platform. We provide evidence that the β‐ketoacyl synthase (KS) domain determines chain length in these mushroom NR‐PKSs, even though their KS domains differ in only ten amino acids. A unique proline‐rich linker connecting the acyl carrier protein with the thioesterase domain varies most between these two enzymes but is not involved in chain length control. Unequal twins: The mushroom polyketide synthase twins CoPKS1 and CoPKS4 differ strongly in their preference for hepta‐ or octaketide production. A series of chimeric CoPKS enzymes identified ten residues in the β‐ketoacyl synthase domain as critical for chain length control, while a unique proline‐rich linker is critical for the overall activity.