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A putative branched-chain-amino-acid transaminase gene required for HC-toxin biosynthesis and pathogenicity in Cochliobolus carbonum

Department of Energy – Plant Research Laboratory, Michigan State University, East Lansing, MI 48824 , USA¹

Author for correspondence: J. D. Walton. Tel: +1 517 353 4885. Fax: +1 517 353 9168. e-mail: walton{at}pilot.msu.edu

The cyclic tetrapeptide HC-toxin is required for pathogenicity of the filamentous fungus Cochliobolus carbonum on maize. HC- toxin production is controlled by a complex locus, TOX2. The isolation and characterization of a new gene of the TOX2 locus, TOXF, is reported. It is shown that TOXF is specifically required for HC-toxin production and pathogenicity. It is present as two or three copies in all HC-toxin-producing (Tox2⁺ ) isolates and is absent in toxin-non-producing strains. The deduced amino acid sequence of TOXF has moderate homology to many known or putative branched-chain-amino-acid transaminases from various species. A strain of C. carbonum with all copies of TOXF disrupted grew normally but lost HC-toxin production and pathogenicity. It is proposed that TOXF has a biosynthetic role in HC-toxin synthesis, perhaps to aminate a precursor of Aeo (2- amino-9,10-epoxi-8-oxodecanoic acid).

Keywords: Cochliobolus (Helminthosporium) carbonum, cyclic peptide, branched-chain-amino-acid transaminase, TOXF, HC-toxin

Abbreviations: Aeo, 2-amino-9,10-epoxi-8-oxodecanoic acid; BAC, bacterial artificial chromosome; BCAT, branched-chain-amino-acid transaminase

The GenBank accession number for the nucleotide sequence reported in this paper is AF157629.

^a Present address: Department of Forest Resources, College of Agriculture and Life Sciences, Konkuk University, Seoul, Korea.

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