The cyclic peptide synthetase catalyzing HC-toxin production in the filamentous fungus Cochliobolus carbonum is encoded by a 15.7-kilobase open reading frame. |
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Authors: | J S Scott-Craig D G Panaccione J A Pocard J D Walton |
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Institution: | Department of Energy-Plant Research Laboratory, Michigan State University, East Lansing 48824. |
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Abstract: | Race 1 of Cochliobolus carbonum, a fungal plant pathogen, owes its exceptional virulence on certain genotypes of maize to the production of HC-toxin, a cyclic tetrapeptide. Production of HC-toxin is controlled by a single known gene, TOX2. Race 1, but not races that do not make HC-toxin, contains two copies of a 22-kilobase (kb) region of chromosomal DNA that is required for HC-toxin biosynthesis and hence virulence. We have sequenced this 22-kb region and here show that it contains an open reading frame of 15.7 kb that encodes a multifunctional cyclic peptide synthetase of potential M(r)574,620. This gene, called HTS1, apparently contains no introns. The predicted gene product, HC-toxin synthetase (HTS), contains four amino acid-binding (adenylate-forming) domains that are highly similar to those found in other cyclic peptide synthetases and other adenylate-binding enzymes. The DNA sequence encodes tryptic peptides derived from two HC-toxin biosynthetic enzymes, HC-toxin synthetase 1 (HTS-1) and HC-toxin synthetase 2 (HTS-2), indicating that these two enzymes exist in vivo as part of a single polypeptide. Consistent with this, in some enzyme preparations antibodies against the enzyme HTS-2, which was originally purified as a protein with a subunit M(r) of 160,000, recognize a protein with an estimated subunit M(r) greater than 480,000. |
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