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

Four Streptomyces species have been described as the causal agents of scab disease, which affects economically important root and tuber crops worldwid e. These species produce a family of cyclic dipeptides, the thaxtomins, whi ch alone mimic disease symptomatology. Structural considerations suggest th at thaxtomins are synthesized non-ribosomally. Degenerate oligonucleotide p rimers were used to amplify conserved portions of the acyladenylation modul e of peptide synthetase genes from genomic DNA of representatives of the fo ur species. Pairwise Southern hybridizations identified a peptide synthetas e acyladenylation module conserved among three species. The complete nucleo tide sequences of two peptide synthetase genes (txtAB) were determined from S. acidiscabies 84.104 cosmid Library clones. The organization of the dedu ced TxtA and TxtB peptide synthetase catalytic domains is consistent with t he formation of N-methylated cyclic dipeptides such as thaxtomins. Based on high-performance liquid chromatography (HPLC) analysis, thaxtomin A produc tion was abolished in txtA gene disruption mutants. Although the growth and morphological characteristics of the mutants were identical to those of th e parent strain, txtA mutants were avirulent on potato tubers. Moreover, in troduction of the thaxtomin synthetase cosmid into a txtA mutant restored b oth pathogenicity and thaxtomin A production, demonstrating a critical role for thaxtomins in pathogenesis.