Type III Polyketide Synthases Responsible for Phenolic Lipid Synthesis
In various microorganisms, phenolic lipids, consisting of a polar aromatic ring and a hydrophobic alkyl chain, are synthesized by type III polyketide synthases (PKSs). In Azotobacter vinelandii, two type III PKSs, ArsB and ArsC, are responsible for the biosynthesis of alkylresorcinols and alkylpyrones, respectively, which are the major lipids in the cyst membrane. In Streptomyces griseus, SrsA is involved in synthesizing alkylquinones, which confer resistance to β-lactam antibiotics. In Mycobacterium smegmatis, PKS10 is involved in the biosynthesis of alkylquinones, which are proposed to act as electron-shuttling molecules. The phenolic lipid-producing type III PKSs are distributed in a wide variety of Gram-positive and Gram-negative bacteria, some fungi, and plants. Thus, phenolic lipids produced by type III PKSs play important, but so far overlooked, roles as minor components in biological membranes and, more importantly, as mobile electron carriers in the respiratory electron transport chain in some bacteria.
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