The txtAB genes of the plant pathogen Streptomyces acidiscabies encode a peptide synthetase required for phytotoxin thaxtomin A production and pathogenicity
Fg. Healy et al., The txtAB genes of the plant pathogen Streptomyces acidiscabies encode a peptide synthetase required for phytotoxin thaxtomin A production and pathogenicity, MOL MICROB, 38(4), 2000, pp. 794-804
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.