Direct transfer of starter substrates from type I fatty acid synthase to type III polyketide synthases in phenolic lipid synthesis

Abstract

Alkylresorcinols and alkylpyrones, which have a polar aromatic ring and a hydrophobic alkyl chain, are phenolic lipids found in plants, fungi, and bacteria. In the Gram-negative bacterium Azotobacter vinelandii, phenolic lipids in the membrane of dormant cysts are essential for encystment. The aromatic moieties of the phenolic lipids in A. vinelandii are synthesized by two type III polyketide synthases (PKSs), ArsB and ArsC, which are encoded by the ars operon. However, details of the synthesis of hydrophobic acyl chains, which might serve as starter substrates for the type III polyketide synthases (PKSs), were unknown. Here, we show that two type I fatty acid synthases (FASs), ArsA and ArsD, which are members of the ars operon, are responsible for the biosynthesis of C 22-C26 fatty acids from malonyl-CoA. In vivo and in vitro reconstitution of phenolic lipid synthesis systems with the Ars enzymes suggested that the C 22-C26 fatty acids produced by ArsA and ArsD remained attached to the ACP domain of ArsA and were transferred hand-to-hand to the active-site cysteine residues of ArsB and ArsC. The type III PKSs then used the fatty acids as starter substrates and carried out two or three extensions with malonylCoA to yield the phenolic lipids. The phenolic lipids in A. vinelandii were thus found to be synthesized solely from malonyl-CoA by the four members of the ars operon. This is the first demonstration that a type I FAS interacts directly with a type III PKS through substrate transfer.Azotobacter vinelandii ͉ alkylresorcinol ͉ alkylpyrone ͉ long-chain fatty acid ͉ cyst

Type III polyketide synthases in natural product biosynthesis

“…ArsB then uses the fatty acyl-ArcAs as the starter substrates and carries out three steps of extension with malonylCoA to yield alkylresorcinols. This is the first demonstration that type I FASs interact directly with a type III PKS through substrate transfer (33). SrsA from S. griseus can also catalyze the formation of alkylresorcinols, in a similar fashion to ArsB.…”

“…For example, the phenolic lipid synthesis systems with the ars enzymes suggest that the C 22 -C 26 fatty acids produced by ArsA and ArsD remained attached to the ACP domain of ArsA and were transferred hand-to-hand to the active-site Cys residues of ArsB and ArsC (33). Moreover, SCO7671 from S. coelicolor could use both CoA-and ACP-tethered starters.…”

Induced biosyntheses of a novel butyrophenone and two aromatic polyketides in the plant pathogen Stagonospora nodorum

“…The two genes are clustered in the chromosome, but without any methyltransferase gene nearby. The polyketide chain transfer between two PKSs was reported for the bacterial PKS system of the alkylresorcinol and alkylpyrone biosynthesis in Azotobacter vinelandii 18 , and for the formation of phlorocaprophenone in Dictyostellium discoidium (type III PKS Steely 2). 19 On the other hand, although (-)-(3R)-mellein methyl ether (3) is widely distributed in fungi, its biosynthetic enzymes have not been reported.…”

Enzymatic synthesis of bis-5-alkylresorcinols by resorcinol-producing type III polyketide synthases

“…One class of the products of type III PKSs includes alkylresorcinols that consist of polar dihydroxybenzene rings and hydrophobic alkyl chains, and exhibit a wide variety of biological and biochemical activities. [2][3][4][5][6] For example, ArsB, a bacterial type III PKS from Azotobacter vinelandii, catalyzes three condensations of malonyl-CoA with a long-chain acyl starter substrate and subsequently cyclizes the resultant tetraketide intermediate by aldol condensation to yield alkylresorcinol. 3,4 ORAS, a fungal type III PKS from Neurospora crassa, synthesizes tetraketide and pentaketide alkylresorcylic acids from a long-chain acyl starter substrate such as stearoyl-CoA.…”

“…[2][3][4][5][6] For example, ArsB, a bacterial type III PKS from Azotobacter vinelandii, catalyzes three condensations of malonyl-CoA with a long-chain acyl starter substrate and subsequently cyclizes the resultant tetraketide intermediate by aldol condensation to yield alkylresorcinol. 3,4 ORAS, a fungal type III PKS from Neurospora crassa, synthesizes tetraketide and pentaketide alkylresorcylic acids from a long-chain acyl starter substrate such as stearoyl-CoA. 5 ARAS2 (Oryza sativa genome DB os10g08620, alkylresorcylic acid synthase 2), a plant type III PKS from the rice plant O. sativa, also produces a tetraketide alkylresorcylic acid from a longchain acyl starter substrate, as does ORAS (our unpublished data).…”