FIGURE 1.

Proposed mechanism for type III PKS reactions. A, phenolic lipid synthesis by two type III PKSs in A. vinelandii. Alkylpyrone formation via an enzyme-bound linear intermediate is depicted, but alkylpyrone can also be formed from the linear intermediate that is released from the enzyme. Two pathways have been suggested for the synthesis of alkylresorcinol by ArsB. In the “hydrolysis first” pathway, aldol cyclization occurs in parallel to decarboxylation (7, 21). In contrast, during the “aldol condensation first” pathway, the cyclization is followed by sequential β-keto decarboxylation and dehydration or by coupled decarboxylation/dehydration (32). B, alkylresorcylic acid synthesis by type III PKSs, including ORAS.