A Baltic Sea microbiome-derived Type III polyketide synthase produces host-protective alkylresorcinols in E. coli

Marine microorganisms are a prolific source of novel polyketide synthases (PKSs) that generate structurally diverse and functional polyketides. Here, we identify a novel type III PKS, ArsMB, from Pontimonas sp. via metagenomic mining of a Baltic Sea microbiome. Heterologous expression of ars MB synthetic gene in an Escherichia coli host that lacks a native type III PKS, combined with a two-step strategy to boost acyl-CoA precursor availability in a glucose-mineral medium, enabled the in vivo synthesis of two cell-associated alkylresorcinols, identified by GC-MS as 5-pentadecyl-1,3-benzenediol (ars1) and 5-tridecyl-1,3-benzenediol (ars2). The product profile is consistent with a tetraketide alkylresorcinol biosynthetic pathway utilizing C16 and C14 acyl starter units, drawn from the host acyl-CoA pool. In agreement, AlphaFold2-based structural modeling predicts a Cys139/His275/Asn308 catalytic triad and a long hydrophobic substrate tunnel compatible with long-chain acyl starter units. E. coli producing the amphipathic cell-associated alkylresorcinols ars1/ars2 exhibited increased resistance to muralytic digestion by endolysins and to hydrogen peroxide. These findings expand the known repertoire of marine-derived type III PKSs and highlight the biotechnological potential of ArsMB-derived alkylresorcinols.