title: A eukaryotic alanine racemase gene involved in cyclic peptide biosynthesis.
abstract: The cyclic tetrapeptide HC-toxin is an essential virulence determinant for the plant pathogenic fungus Cochliobolus carbonum and an inhibitor of histone deacetylase. The major form of HC-toxin contains the D-isomers of Ala and Pro. The non-ribosomal peptide synthetase that synthesizes HC-toxin has only one epimerizing domain for conversion of L-Pro to D-Pro; the source of D-Ala has remained unknown. Here we present the cloning and characterization of a new gene involved in HC-toxin biosynthesis, TOXG. TOXG is present only in HC-toxin-producing (Tox2(+)) isolates of C. carbonum. TOXG is able to support D-Ala-independent growth of a strain of Escherichia coli defective in D-Ala synthesis. A C. carbonum strain with both of its copies of TOXG mutated grows normally in culture, and although it no longer makes the three forms of HC-toxin that contain D-Ala, it still makes a minor form of HC-toxin that contains Gly in place of D-Ala. The addition of D-Ala to the culture medium restores production of the D-Ala-containing forms of HC-toxin by the toxG mutant. The toxG mutant has only partially reduced virulence. It is concluded that TOXG encodes an alanine racemase whose function is to synthesize D-Ala for incorporation into HC-toxin.
authors.affiliation: Department of Energy Plant Research Laboratory and Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 28824, USA.
authors.forename: Y Q
authors.initials: YQ
authors.lastname: Cheng
authors.forename: J D
authors.initials: JD
authors.lastname: Walton
mesh_terms: D000411:Alanine Racemase / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D001203:Ascomycota / Q000201:enzymology* / Q000472:pathogenicity; D001483:Base Sequence; D002876:Chromosomes, Bacterial; D004274:DNA, Recombinant; D008969:Molecular Sequence Data; D010456:Peptides, Cyclic / Q000096:biosynthesis*; D010641:Phenotype
chemical_list: D004274:DNA, Recombinant; D010456:Peptides, Cyclic; D000411:Alanine Racemase