In vitro Characterization of Chlorophenolic Building Block Formation in Ambigol Biosynthesis

The cyanobacterium Fischerella ambigua is a natural producer of polychlorinated aromatic compounds, the ambigols A-E. The biosynthetic gene cluster (BGC) of these highly halogenated triphenyls has been recently identified by heterologous expression. It consists of 10 genes named ab1-10. Two of the encoded enzymes, i.e. Ab2 and Ab3, were identified by in vitro and vivo assays as cytochrome P450 enzymes responsible for biaryl and biaryl ether formation. Here, the biosynthetic steps leading towards the substrates of these P450 enzymes were investigated by in vitro assays. Ab7, an isoenzyme of a 3-deoxy-7-phosphoheptulonate (DAHP) synthase is involved in chorismate biosynthesis by the shikimate pathway. Chorismate in turn is further converted by a dedicated chorismate lyase (Ab5) yielding 4-hydroxybenzoicacid (4-HBA). The stand alone adenylation domain Ab6 is necessary to activate 4-HBA, which is subsequently tethered to the acyl carrier protein (ACP) Ab8. The ACP (Ab8) domain bound substrate is chlorinated by Ab10 in meta position yielding 3-Cl-4-HBA, which is then transfered by the condensation domain (C) to the peptidyl carrier protein and released by the TE domain of Ab9. The released product is then expected to be the dedicated substrate of the halogenase Ab1 producing the monomeric ambigol building block 2,4-dichlorophenol.