cis-Δ^2,3-Double Bond of Phoslactomycins Is Generated by a Post-PKS Tailoring Enzyme

Abstract

The antifungal phoslactomycins (PLM A-F), produced by Streptomyes sp HK803, are structurally unusual in that three of their four double bonds are in the cis form (Δ 12,13 , Δ 14,15 , Δ 2,3 ). The PLM polyketide synthase (PKS) has the predicted dehydratase catalytic domain in modules 1,2 and 5 required for establishing two of these cis double bonds (Δ 12,13 Δ 14,15 ), as well as the only trans Δ 6,7 double bond. By contrast, the formation of the cis Δ 2,3 in the unsaturated lactone moiety of PLMs has presented an enigma because the predicted dehydratase domain in module 7 is absent. Herein, we have demonstrated that the plmT 2 gene product, with no homology to PKS dehydratase domains, is required for efficient formation of the cis Δ 2,3 alkene. A series of new PLM products in which the C3 hydroxyl group is retained, are made in plmT 2 deletion mutants. In all of these cases, however, the hydroxyl group is esterified with malonic acid. These malonylated PLM products are converted to the corresponding cis Δ 2,3 PLM products and acetic acid by a facile base-catalyzed decarboxylative elimination reaction. Complete or partial restoration of natural PLM production in a plmT 2 deletion mutant can be accomplished by plasmid based expression of plmT 2 or fos ORF4 (a homologous gene from the fostriecin biosynthetic gene cluster), respectively. The data indicate that dehydrataseindependent pathways also function in establishment of unsaturated 6-membered lactone moieties in other PKS pathways, and provide the first biosynthetic insights into the possible routes by which unusual malonylated polyketide products are generated.Modular polyketide synthases (PKSs) generate a vast array of structurally diverse natural products with important biological activities. 1 They are responsible for formation of macrolides, the vast majority of which contain one or more double bonds. 2 The majoritiy of other natural produces made by modular PKSs similarly contain some double bonds. In the majority of cases the alkenes in the polyketide product are in the trans form. The formation of these double bonds has been shown to be directly attributed to the presence of a ketoreductasedehydratase (KR-DH) didomain within the appropriate module. 3, 4 These didomains catalyze a 3-keto reduction and subsequent Δ 2,3 elimination reaction with the PKS-tethered 3-ketoacyl polyketide intermediates.The phoslactomycins (PLMs A-F Figure 1) and fostriecin belong to a class of phosphorylated polyketides that contain multiple double bonds in the cis form. 5, 6 For the PLMs there are three double bonds in the cis form (Δ 12,13 , Δ 14,15 , Δ 2,3 ) and one in the trans form (Δ 6,7 ). 7, 8The PLM biosynthetic gene cluster has been cloned and sequenced and shown to encode a modular PKS. 9 Modules 1 and 2 contain the expected dehydratase DH-KR didomain required for formation of the conjugated diene, while module 5 contains a DH-KR domain likely responsible for formation of the trans Δ 6,7 alkene. The KR-DH domains which generate a trans double bond do so via a...

Functional Analysis of the Fusarielin Biosynthetic Gene Cluster

“…The cis double bond could arise from the Diels-Alder reaction or from the actions of enoyl-isomerase or reduction domains in modular PKS's during chain elongation [16][17][18]. The trans-to-cis isomerization can also be catalyzed by exogenous post PKS tailoring enzymes, as observed in phoslactomycin biosynthesis in Streptomyces, which is catalyzed by an enzyme with homology to a nicotinamide adenine dinucleotide (NAD)-dependent epimerase/dehydratase [19]. FSL3 can play a similar role in fusarielin biosynthesis to form the C11=C12 cis double bond by moving a hypothetical C10=C11 or C12=C13 trans double bond.…”

Die biosynthetische Grundlage der Polyketid‐Vielfalt

“…Die cis-konfigurierte a,b-Doppelbindung im Lacton-Rest von Phoslactomycin (57) wird durch eine Post-PKS-Reaktion eingeführt. [181] Weitere Beispiele, in denen modulare Polyketid-Synthasen mehrfache cis-Doppelbindungen erzeugen, sind die Biosynthesewege, die zu den myxobakteriellen Metaboliten Chivosazol A (78) [182] und Disorazol A1 (79) führen (Abbildung 3). [183] Es sollte jedoch erwähnt werden, dass der vorhergesagte stereochemische Verlauf der Reduktion in vielen Fällen nicht in Einklang mit der endgültigen Konfiguration der Doppelbindung ist, weshalb sich die Frage stellt, wie die Konfigurationen der Doppelbindungen bestimmt werden oder ob der Aminosäurecode wirklich in allen Fällen zur Vorhersage geeignet sind.…”

Polyene macrolide biosynthesis in streptomycetes and related bacteria: recent advances from genome sequencing and experimental studies

“…In polyketides, cis double bonds are less common. Some of these result from trans-cis isomerisation (Perlova et al, 2006;Vergnolle et al, 2011) or late modification (Palaniappan et al, 2008), others from dehydration of 3L-3-hydroxyacyl chains by the PKS. In these last cases, DH domains are paired with A-type KRs (Alhamadsheh et al, 2007;Labonte and Townsend, 2013).…”

“…This suggests that as microbial genome sequencing reveals increasing numbers of PKSs, it should be possible to infer initial double bond geometry in cryptic products, using existing prediction methods. However, cis alkenes formed as a result of post-PKS reactions remain unpredictable (Palaniappan et al, 2008;Vergnolle et al, 2011).…”