Genetic Manipulation of PKS Genes

The three domains involved in 3-carbonyl processing function in the hierarchical order KR-DH-ER. Thus, if all three are present in a given domain, loss of KR function obviates the requirement for the DH and ER functions, and loss of the DH domain renders the otherwise functional ER domain unnecessary. Loss of the KR domain results in the appearance of a keto group in place of the original hydroxyl. Loss of the DH domain leaves the hydroxyl side chain in place of the methylene function, if a functional ER domain is present within the affected module if an ER domain is not present, the olefin formerly present at the site in the polyketide is replaced by a hydroxyl residue. 

The formation of avermectin aglycons involves KRs that resemble type I and II FAS. The C5-keto group is reduced by a monofunctional KR (AveF) that is not within module 11 of the AVES 4 (Fig. 10). The mutants defective in C5 ketore-ductase gene, aveF, which were obtained by introducing an insertion mutation in the promoter region of aveD-aveF operon produced 5-oxoavermectins [104]. 

Since the stracture of the side chain at a given site in the polyketide chain is determined by the structure of the extender unit incorporated at the corresponding site in the nascent chain, changes in the side chains that appear in the final product will result from replacement of one extender unit for another. Because the AT domains determine the nature of the cognate extender unit incorporated (described in Section 111), AT swapping results in corresponding side-chain changes. 

The starter unit, acetyl-CoA, propionyl-CoA, or the other acyl-CoA, is also determined by an AT (or ligase in rapamycin and FK520 PKSs) function present at the first functional domain of the N terminus of the PKS polypeptide. Domain swaps at these sites can result in changes in the starting side chain. 

O Hagan, D. (1991). The Polyketide Metabolites. Ellis Horwood, Chichester, U.K. 

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