The txtAB genes of the plant pathogen Streptomyces acidiscabies encode a peptide synthetase required for phytotoxin thaxtomin A production and pathogenicity

Authors

  • Frank G. Healy,

    1. Department of Plant Pathology, 334 Plant Science Building, Cornell University, Ithaca, NY 14853, USA.
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  • Michael Wach,

    1. Department of Plant Pathology, 334 Plant Science Building, Cornell University, Ithaca, NY 14853, USA.
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  • Stuart B. Krasnoff,

    1. USDA Agricultural Research Service, US Plant, Soil, and Nutrition Laboratory, Tower Road, Ithaca, NY 14853, USA.
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  • Donna M. Gibson,

    1. USDA Agricultural Research Service, US Plant, Soil, and Nutrition Laboratory, Tower Road, Ithaca, NY 14853, USA.
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  • Rosemary Loria

    Corresponding author
    1. Department of Plant Pathology, 334 Plant Science Building, Cornell University, Ithaca, NY 14853, USA.
    • *For correspondence. E-mail rl21@ cornell.edu; Tel. (+1) 607 255 7831; Fax (+1) 607 255 4471. †Present address: Department of Microbiology and Cell Science, Box 110700, Museum Road, University of Florida, Gainesville, FL 32611-0700, USA.

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Abstract

Four Streptomyces species have been described as the causal agents of scab disease, which affects economically important root and tuber crops worldwide. These species produce a family of cyclic dipeptides, the thaxtomins, which alone mimic disease symptomatology. Structural considerations suggest that thaxtomins are synthesized non-ribosomally. Degenerate oligonucleotide primers were used to amplify conserved portions of the acyladenylation module of peptide synthetase genes from genomic DNA of representatives of the four species. Pairwise Southern hybridizations identified a peptide synthetase acyladenylation module conserved among three species. The complete nucleotide sequences of two peptide synthetase genes (txtAB) were determined from S. acidiscabies 84.104 cosmid library clones. The organization of the deduced TxtA and TxtB peptide synthetase catalytic domains is consistent with the formation of N-methylated cyclic dipeptides such as thaxtomins. Based on high-performance liquid chromatography (HPLC) analysis, thaxtomin A production was abolished in txtA gene disruption mutants. Although the growth and morphological characteristics of the mutants were identical to those of the parent strain, txtA mutants were avirulent on potato tubers. Moreover, introduction of the thaxtomin synthetase cosmid into a txtA mutant restored both pathogenicity and thaxtomin A production, demonstrating a critical role for thaxtomins in pathogenesis.

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