Involvement of β‐Alkylation Machinery and Two Sets of Ketosynthase‐Chain‐Length Factors in the Biosynthesis of Fogacin Polyketides in Actinoplanes missouriensis

Abstract

Fogacin and two novel fogacin derivatives, fogacins B and C, were isolated from the rare actinomycete Actinoplanes missouriensis. Biosynthesis of fogacin C apparently requires β alkylation of a polyketide chain. The fogacin biosynthetic type II polyketide synthase (PKS) gene cluster contains a hydroxymethylglutaryl‐coenzyme A synthase (HCS) cassette, which is usually responsible for β alkylation in the type I PKS system. Another characteristic of the fog cluster is that it encodes two sets of ketosynthase (KS) and chain‐length factor (CLF). Inactivation of either of the two KS genes in A. missouriensis and heterologous expression of the HCS cassette with either of the two KS‐CLF genes in Streptomyces albus indicated that each KS‐CLF had a different starter substrate specificity: one preferred an unusual β‐alkylated starter and the other preferred a normal acetyl starter. This study expands knowledge of HCS cassette‐dependent β alkylation into the type II PKS system and provides a natural example of combinatorial biosynthesis for producing diverse polyketides from different starter substrates.

Polyketide β-branching: diversity, mechanism and selectivity

“…Since these early reports, many natural products possessing b-branches have been isolated, mostly commonly from trans-AT PKSs, 1,2 but also cis-AT and type II PKSs as well. 30,31 The mechanism for b-branch formation in the biosynthesis of bacillaene was elucidated in vitro by ( Fig. 2A).…”

“…14). 31 The core polyketide backbone of fogacin C is typical of aromatic polyketides produced by actinomycetes, whereby basic chain assembly is catalysed by a minimal PKS comprised of a KS, a chain length (or initiation) factor (CLF) and an ACP. However, the structure contains a novel tertiary alkene b-branch arising from the incorporation of a methylmalonyl building block.…”

“…Homologous domains have since been identied in the production of linear and bbranched starter units in the type II PKS that produce lomaiviticin and fogacin C respectively (see Section 5.5). 31 The HMGS-catalysed conversion of propionyl-ACP D and bketothioester-ACP A produces a g-methyl substituted HMG-ACP A that then undergoes LnmF-catalysed dehydration. The gene cluster lacks an ECH 2 domain but instead alternate processing takes place on the newly-installed b-branch.…”

Structures of LnmK a Bifunctional Acyltransferase/Decarboxylase with Substrate Analogs Reveals Basis for Selectivity and Stereospecificity

“…LnmK and homologs are bifunctional acyltransferase/decarboxylases (AT/DC) that generate propionyl-S-acyl-carrier protein (ACP) for incorporation by diverse polyketide synthases (PKS) into natural products, such as leinamycin, lomaiviticin, myxovirescein, nenestatin and fogacin. [1][2][3][4] LnmK accepts (2R)-methylmalonyl-CoA (D-configuration) as a substrate, whereas ATs of canonical modular type I PKS, like deoxyerythronolide B synthase, accept (2S)-methylmalonyl-CoA (L-configuration). 5,6 This opposite stereochemical preference provides a route to the selective incorporation of novel 2-substituted malonyl-thioester extender units, Figure 1.…”

Structures of LnmK a Bifunctional Acyltransferase/Decarboxylase with Substrate Analogs Reveals Basis for Selectivity and Stereospecificity

“…LnmK and homologs are bifunctional acyltransferase/decarboxylases (AT/DC) that generate propionyl-S-acyl-carrier protein (ACP) for incorporation by diverse polyketide synthases (PKS) into natural products, such as leinamycin, lomaiviticin, myxovirescein, nenestatin and fogacin. [1][2][3][4] LnmK accepts (2R)-methylmalonyl-CoA (D-configuration) as a substrate, whereas ATs of canonical modular type I PKS, like deoxyerythronolide B synthase, accept (2S)-methylmalonyl-CoA (L-configuration). 5,6 This opposite stereochemical preference provides a route to the selective incorporation of novel 2-substituted malonyl-thioester extender units, Figure 1.…”