Abstract
Pseudomonas syringae is a phytopathogenic bacterium widely spread on terrestrial plants. Sulfodiaminophosphinyl tripeptide Phaseolotoxins (PHTs), produced by P. syringae pv. phaseolicola and P. syringae pv. actinidiae, represent a kind of antimetabolic phytotoxins. PHTs inhibit host cell Ornithine transcarbamylase (OTCase) activity and induce Arginine auxotrophic phenotype. The biosynthesis of PHT is temperature dependent, being optically produced at around 18 °C, while blocked above 28 °C. PHT resistant OTCase ArgK acts as a functional replacement of housekeeping OTCase ArgF, which is the acting target of PHT, to confer PHT producers with self-resistance. It was postulated that argK might be regulated directly by a PHT biosynthetic precursor and indirectly by temperature with an unknown manner. Neither transcriptional regulator nor thermal regulation related protein encoding gene was detected from PHT biosynthetic gene cluster. The tripeptide, Cit-Ala-hArg, was identified to be a by-product of PHT biosynthetic pathway in this report. Formation of Cit-Ala-hArg was catalyzed by ArgK with tripeptide Orn-Ala-hArg and carbamyl phosphate as substrates. It showed that ArgK not only provided alternative Arginine source as reported previously, but also controlled the production of PHTs by converting PHT biosynthetic precursors to nontoxic Cit-Ala-hArg reservoir for producers’ self-defense.
Highlights
Pseudomonas syringae is a phytopathogenic bacterium widely spread on terrestrial plants
Precursor ion scan (PIS) mass spectrometry, feeding experiments and enzyme assay were employed in the identification of PHT biosynthetic pathway by-products, tripeptides Cit-Ala-hArg, which indicating a dual role of ArgK for PHT producers self-defense
PHTs were detected in the cultural supernatants of wild type strain P. syringae pv. phaseolicola 1448A by High resolution mass spectrometry (HR MS) and 31P nuclear magnetic resonance (NMR) analysis
Summary
Pseudomonas syringae is a phytopathogenic bacterium widely spread on terrestrial plants. Formation of Cit-Ala-hArg was catalyzed by ArgK with tripeptide Orn-Ala-hArg and carbamyl phosphate as substrates It showed that ArgK provided alternative Arginine source as reported previously, and controlled the production of PHTs by converting PHT biosynthetic precursors to nontoxic Cit-Ala-hArg reservoir for producers’ self-defense. PHTs are competitive inhibitors to mammalian and bacterial OTCase, including the OTCase ArgF of PHT producing strain P. syringae pv. Phaseolicola cells, ArgK acts as a functional replacement of housekeeping OTCase ArgF to provide an alternative Arginine source whenever ArgF is inhibited by PHTs3,16. Precursor ion scan (PIS) mass spectrometry, feeding experiments and enzyme assay were employed in the identification of PHT biosynthetic pathway by-products, tripeptides Cit-Ala-hArg, which indicating a dual role of ArgK for PHT producers self-defense
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