Submitted to: Molecular Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2000
Publication Date: N/A
Interpretive Summary: Streptomycetes are soilborne bacteria that produce the vast majority of commercially important antibiotics, enzymes, and other compounds. Among the hundreds of described Streptomyces species, only four species are plant pathogens (S. scabies, S. acidiscabies, S. turgidiscabies and S. ipomoeae), and they are the causal agents of scab diseases of economically important root and tuber crops, such as potatoes. These organisms produce a small chemical compound called thaxtomin which causes disease symptoms of scab on susceptible crops. This work was undertaken to identify the structural genes that produce thaxtomin. With the identification of the genes responsible for thaxtomin biosynthesis, it will now be possible to more precisely assess the importance of this compound in causing plant disease.
Technical Abstract: Streptomycetes are renowned for the production of enzymes and metabolites of pharmaceutical, agricultural, and industrial importance. 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 unusually modified highly phytotoxic cyclic dipeptides, the thaxtomins, which alone mimic disease symptomatology. Many small microbially-derived peptide metabolites are produced non-ribosomally on large multifunctional peptide synthetases; structural considerations suggest thaxtomins are synthesized non-ribosomally. Degenerate oligonucleotide primers were employed to amplify conserved portions of the acyladenylation module of peptide synthetase genes from genomic DNA of representatives of the four plant pathogens. Pairwise Southern hybridizations identified a peptide synthetase eacyladenylation 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 HPLC analysis, thaxtomin A production was abolished in mutants derived from targeted gene disruption of the first peptide synthetase gene (txtA). Further analysis of regions surrounding txtAB may reveal genes involved in modification of the cyclic dipeptide, including the addition of the nitro group, a functional group seldom seen in biological systems, to L-tryptophan.