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J Biol Chem. 2000 Feb 18;275(7):4906-11.

A eukaryotic alanine racemase gene involved in cyclic peptide biosynthesis.

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  • 1Department of Energy Plant Research Laboratory and Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 28824, USA.


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.

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