Streptomyces scabies is an important causative agent of common scab disease of potato tubers and other root crops. The primary
virulence factor produced by this pathogen is a phytotoxic secondary metabolite called
thaxtomin A, which is essential for disease development. In addition, the genome of S.
scabies harbors a virulence-associated biosynthetic gene cluster called the
coronafacic acid (CFA)-like gene cluster, which was previously predicted to produce metabolites that resemble the Pseudomonas syringae
coronatine (COR) phytotoxin. COR consists of CFA linked to an ethylcyclopropyl
amino acid called
coronamic acid, which is derived from L-allo-
isoleucine. Using a combination of genetic and chemical analyses, we show that the S.
scabies CFA-like gene cluster is responsible for producing CFA-
L-isoleucine as the major product as well as other minor COR-like metabolites. Production of the metabolites was shown to require the cfl gene, which is located within the CFA-like gene cluster and encodes an
enzyme involved in ligating CFA to its
amino acid partner. CFA-
L-isoleucine purified from S.
scabies cultures was shown to exhibit bioactivity similar to that of COR, though it was found to be less toxic than COR. This is the first report demonstrating the production of coronafacoyl phytotoxins by S.
scabies, which is the most prevalent scab-causing pathogen in North America.