An Iterative Module in the Azalomycin F Polyketide Synthase Contains a Switchable Enoylreductase Domain

Abstract

Detailed analysis of the modular Type I polyketide synthase (PKS) involved in the biosynthesis of the marginolactone azalomycin F in mangrove Streptomyces sp. 211726 has shown that only nineteen extension modules are required to accomplish twenty cycles of polyketide chain elongation. Analysis of the products of a PKS mutant specifically inactivated in the dehydratase domain of extension‐module 1 showed that this module catalyzes two successive elongations with different outcomes. Strikingly, the enoylreductase domain of this module can apparently be “toggled” off and on : it functions in only the second of these two cycles. This novel mechanism expands our understanding of PKS assembly‐line catalysis and may explain examples of apparent non‐colinearity in other modular PKS systems.

Identification of a biosynthetic gene cluster for the polyene macrolactam sceliphrolactam in a Streptomyces strain isolated from mangrove sediment

“…It was recently found that the biosynthesis of azalomycin involves a similar iterative module. This iterative module contains an ER domain whose activity can be “toggled” off and on to ensure that the ER domain only functions in the second extension cycle 31 , 42 . The selective keto-reduction on different polyketide intermediates by the SceQ or SceR module is also reminiscent of the programmed keto-reduction mechanism observed in partially reducing iterative PKSs 32 , 43 – 45 .…”

Diversity of Polyketide Chains Achieved by Deleting the Tailoring Genes in the Biosynthesis of Ansatrienins

“…A further exception is the production of compound 3 with a shrunken octaketide backbone, which indicates that a specific module is missing during chain elongation. In ring‐size modification of type I PKS, domain skipping or reusing strategies have been well studied . The skipping of either module 5, 6, or 7 could lead to the production of compound 3 .…”

“…In ring-size modification of type IP KS, domain skipping or reusing strategies have been well studied. [17,18] The skippingo fe ither module 5, 6, or 7c ould lead to the production of compound 3.O nt he otherh and, it is now clear that the collinearity rule does not hold for al arge proportion of assembly lines, mostly because of the existence of trans-acting enzymes. [19] Remarkably, there is an extra tridomain polyketide synthase encoded by the orf1 gene in the ast clustert hat is absent in the myc cluster.T his enzymem ay act in trans throughi nteraction with module 7t o releaset he immature octaketide, which subsequently undergoes cyclization by the amide synthase to afford the ansadienin, compound 3 (Scheme 2C), and this is supported by the presenceo fo ther free-standingp roteins involved in polyketide biosynthesis, for example, trans-AT, trans-KR, and trans-AT-ACP.…”

Engineering actinomycetes for biosynthesis of macrolactone polyketides

“…One of the prominent examples is trans -AT or AT-less biosynthesis of polyketides whereas AT activity is provided at each elongation step in trans by free-standing AT usually encoded in the biosynthetic gene clusters [30]. Trans -ER has been characterized from lovastatin polyketide biosynthesis [31] and switchable ER domain has been reported in azalomycin F biosynthesis [32]. Similarly, some trans -acting thioesterases (TE type II) are responsible for editing function by hydrolytic removal of aberrant residues blocking the megasynthase, participation in substrate selection, intermediate, and product release [33].…”