title: Two polyketide synthase-encoding genes are required for biosynthesis of the polyketide virulence factor, T-toxin, by Cochliobolus heterostrophus.
abstract: Cochliobolus heterostrophus race T, causal agent of southern corn leaf blight, requires T-toxin (a family of C35 to C49 polyketides) for high virulence on T-cytoplasm maize. Production of T-toxin is controlled by two unlinked loci, Tox1A and Tox1B, carried on 1.2 Mb of DNA not found in race O, a mildly virulent form of the fungus that does not produce T-toxin, or in any other Cochliobolus spp. or closely related fungus. PKS1, a polyketide synthase (PKS)-encoding gene at Tox1A, and DEC1, a decarboxylase-encoding gene at Tox1B, are necessary for T-toxin production. Although there is evidence that additional genes are required for T-toxin production, efforts to clone them have been frustrated because the genes are located in highly repeated, A+T-rich DNA. To overcome this difficulty, ligation specificity-based expression analysis display (LEAD), a comparative amplified fragment length polymorphism/gel fractionation/capillary sequencing procedure, was applied to cDNAs from a near-isogenic pair of race T (Tox1+) and race O (Tox1-) strains. This led to discovery of PKS2, a second PKS-encoding gene that maps at Tox1A and is required for both T-toxin biosynthesis and high virulence to maize. Thus, the carbon chain of each T-toxin family member likely is assembled by action of two PKSs, which produce two polyketides, one of which may act as the starter unit for biosynthesis of the mature T-toxin molecule.
authors.affiliation: Torrey Mesa Research Institute, Syngenta, 3115 Merryfield Row, San Diego, CA 92121, USA.
authors.forename: Scott E
authors.initials: SE
authors.lastname: Baker
authors.forename: Scott
authors.initials: S
authors.lastname: Kroken
authors.forename: Patrik
authors.initials: P
authors.lastname: Inderbitzin
authors.forename: Thipa
authors.initials: T
authors.lastname: Asvarak
authors.forename: Bi-Yu
authors.initials: BY
authors.lastname: Li
authors.forename: Liang
authors.initials: L
authors.lastname: Shi
authors.forename: O C
authors.initials: OC
authors.lastname: Yoder
authors.forename: B Gillian
authors.initials: BG
authors.lastname: Turgeon
mesh_terms: D001203:Ascomycota / Q000201:enzymology* / Q000472:pathogenicity; D002874:Chromosome Mapping; D032461:Chromosomes, Plant / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D017353:Gene Deletion; D020869:Gene Expression Profiling; D018942:Macrolides / Q000378:metabolism*; D008826:Microbial Sensitivity Tests; D009097:Multienzyme Complexes; D009183:Mycotoxins / Q000096:biosynthesis* / Q000737:chemistry; D010802:Phylogeny; D048630:Polyketide Synthases / Q000737:chemistry / Q000235:genetics*; D036226:Seedlings / Q000382:microbiology; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D037521:Virulence Factors / Q000096:biosynthesis*; D003313:Zea mays / Q000033:anatomy & histology / Q000382:microbiology
chemical_list: D018076:DNA, Complementary; D018942:Macrolides; D009097:Multienzyme Complexes; D009183:Mycotoxins; D037521:Virulence Factors; D048630:Polyketide Synthases