Regulation of coronafacoyl phytotoxin production in the potato common scab pathogen Streptomyces scabies

Cheng, Zhenlong (2018) Regulation of coronafacoyl phytotoxin production in the potato common scab pathogen Streptomyces scabies. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The genus Streptomyces consists of hundreds of species of Gram-positive, filamentous bacteria, which have a complex developmental life cycle. Many specialized metabolites of clinical, agricultural and biotechnological value are produced by Streptomyces. Very few Streptomyces species have been shown to be pathogenic to plants. The best characterized pathogenic species is Streptomyces scabies, which is the main causative agent of potato common scab (CS) disease. CS is characterized by the formation of lesions on the potato tuber surface, which negatively impact the market value of the affected potatoes, leading to significant losses for growers. S. scabies and other CS-causing pathogens produce thaxtomin A, the key virulence factor involved in CS disease. S. scabies also produces N-coronafacoyl-L-isoleucine (CFAL- Ile), which is a member of the coronafacoyl family of phytotoxins. The coronafacoyl phytotoxin biosynthetic gene cluster in S. scabies consists of 15 genes, of which 13 are enzyme-encoding genes and are co-transcribed. The remaining two genes are divergently co-transcribed from the biosynthetic genes. The first gene, scab79591/cfaR, encodes a PAS-LuxR family regulator that activates the transcription of the enzyme-coding genes. The second gene, scab79581/orf1, encodes a ThiF family protein of unknown function. This thesis examines the regulation of CFA-L-Ile biosynthesis in S. scabies and the role of the CfaR and ORF1 proteins. Our results show that CfaR is the principle regulator controlling expression of the coronafacoyl phytotoxin biosynthetic genes and CFA-L-Ile production, while ORF1 augments phytotoxin production in a CfaR dependent manner and may function as a “helper” of CfaR. Bioinformatics analysis suggests that ORF1 may catalyze AMPylation of an unknown target molecule. In addition, this thesis addresses the effects of the plant-derived molecules cellobiose and suberin on CFA-L-Ile production as well as the role of CFA-L-Ile in controlling its own production. While cellobiose and suberin both induce thaxtomin A production, our results suggest that these molecules inhibit CFA-L-Ile production in S. scabies. In vitro binding assays showed that CFA-L-Ile inhibits the binding of CfaR to its target site in the coronafacoyl phytotoxin biosynthetic gene cluster. This suggests that CFA-L-Ile production is subjected to negative feedback regulation in S. scabies.

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