title: Chain initiation on the soraphen-producing modular polyketide synthase from Sorangium cellulosum.
abstract: BACKGROUND
  Polyketides are structurally diverse natural products with a wide range of useful activities. Bacterial modular polyketide synthases (PKSs) catalyse the production of non-aromatic polyketides using a different set of enzymes for each successive cycle of chain extension. The choice of starter unit is governed by the substrate specificity of a distinct loading module. The unusual loading module of the soraphen modular PKS, from the myxobacterium Sorangium cellulosum, specifies a benzoic acid starter unit. Attempts to design functional hybrid PKSs using this loading module provide a stringent test of our understanding of PKS structure and function, since the order of the domains in the loading and first extension module is non-canonical in the soraphen PKS, and the producing strain is not an actinomycete.
  
  
  RESULTS
  We have constructed bimodular PKSs based on DEBS1-TE, a derivative of the erythromycin PKS that contains only extension modules 1 and 2 and a thioesterase (TE) domain, by substituting one or more domains from the soraphen PKS. A hybrid PKS containing the soraphen acyltransferase domain AT1b instead of extension acyltransferase domain AT1 produced triketide lactones lacking a methyl group at C-4, as expected if AT1b catalyses the addition of malonyl-CoA during the first extension cycle on the soraphen PKS. Substitution of the DEBS1-TE loading module AT domain by the soraphen AT1a domain led to the production of 5-phenyl-substituted triketide lactone, as well as the normal products of DEBS1-TE. This 5-phenyl triketide lactone was also the product of a hybrid PKS containing the entire soraphen PKS loading module as well as part of its first extension module. Phenyl-substituted lactone was only produced when measures were simultaneously taken to increase the intracellular supply of benzoyl-CoA in the host strain of Saccharopolyspora erythraea.
  
  
  CONCLUSIONS
  These results demonstrate that the ability to recruit a benzoate starter unit can be conferred on a modular PKS by the transfer either of a single AT domain, or of multiple domains to produce a chimaeric first extension module, from the soraphen PKS. However, benzoyl-CoA needs to be provided within the cell as a specific precursor. The data also support the respective roles previously assigned to the adjacent AT domains of the soraphen loading/first extension module. Construction of such hybrid actinomycete-myxobacterial enzymes should significantly extend the synthetic repertoire of modular PKSs.
authors.affiliation: Cambridge Centre for Molecular Recognition and Department of Biochemistry, University of Cambridge, UK.
authors.forename: C J
authors.initials: CJ
authors.lastname: Wilkinson
authors.forename: E J
authors.initials: EJ
authors.lastname: Frost
authors.forename: J
authors.initials: J
authors.lastname: Staunton
authors.forename: P F
authors.initials: PF
authors.lastname: Leadlay
mesh_terms: D000595:Amino Acid Sequence; D006571:Heterocyclic Compounds; D007700:Kinetics; D018942:Macrolides; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D009231:Myxococcales / Q000201:enzymology*; D010442:Peptide Chain Initiation, Translational; D015202:Protein Engineering; D017434:Protein Structure, Tertiary / Q000235:genetics; D013379:Substrate Specificity
chemical_list: D006571:Heterocyclic Compounds; D018942:Macrolides; D009097:Multienzyme Complexes; C086151:soraphen A