cis-Δ2,3-Double Bond of Phoslactomycins Is Generated by a Post-PKS Tailoring Enzyme

摘要

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 (Δ12,13, Δ14,15, Δ2,3). 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 (Δ12,13Δ14,15), as well as the only trans Δ6,7double bond. By contrast, the formation of the cis Δ2,3 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 plmT2 gene product, with no homology to PKS dehydratase domains, is required for efficient formation of the cis Δ2,3 alkene. A series of new PLM products in which the C3 hydroxyl group is retained, are made in plmT2 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 Δ2,3 PLM products and acetic acid by a facile base-catalyzed decarboxylative elimination reaction. Complete or partial restoration of natural PLM production in a plmT2deletion mutant can be accomplished by plasmid based expression of plmT2 or fos ORF4 (a homologous gene from the fostriecin biosynthetic gene cluster), respectively. The data indicate that dehydratase-independent 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.