2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine, an effective peptide antibiotic from the epiphyte Pantoea agglomerans 48b/90

Abstract

The epiphyte Pantoea agglomerans 48b/90, which has been isolated from soybean leaves, belongs to the Enterobacteriaceae, as does the plant pathogen Erwinia amylovora, which causes fire blight on rosaceous plants such as apples and leads to severe economic losses. Since P. agglomerans efficiently antagonizes phytopathogenic bacteria, the P. agglomerans strain C9-1 is used as a biocontrol agent (BlightBan C9-1). Here we describe the bioassay-guided isolation of a peptide antibiotic that is highly active against the plant pathogen E. amylovora and pathovars of Pseudomonas syringae, and we elucidate its structure. Bioassay-guided fractionation using anion-exchange chromatography followed by hydrophobic interaction liquid chromatography yielded the bioactive, highly polar antibiotic. The compound was identified as 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine by using high-resolution electrospray ionization mass spectrometry and nuclear magnetic resonance techniques. This peptide was found to be produced by three of the nine P. agglomerans strains analyzed. Notably, the biocontrol strain P. agglomerans C9-1 also produces 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine. Previously, 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine has been characterized only from Serratia plymuthica. 2-Amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine has been shown to inhibit the growth of the human pathogen Candida albicans efficiently, but its involvement in the defense of epiphytes against phytopathogenic bacteria has not been investigated so far.

FIG. 1.

Temperature dependence of antibiotic production by Pa48b in liquid PIPES medium. Cultures were analyzed at an OD₅₇₈ of 5.5. Bars represent means from three independent experiments; error bars, standard deviations. Different letters indicate significant differences (P ≤ 0.05).

FIG. 2.

Growth curve and antibiotic production of Pa48b in liquid PIPES medium at 10°C. Bars represent means from three independent experiments; error bars, standard deviations.

FIG. 3.

Identification of 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine from Pa48b as an antibiotic active against E. amylovora. (A) ESI-MS and ESI-MS-MS spectra of purified 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine. TIC, total ion current. (B and C) ¹H NMR and ¹³C NMR spectra, respectively, of 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine. (D) Structure of 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine. (E) Results of agar diffusion assays (on 5b agar) against E. amylovora Ea7 with 3.5 μg/μl amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine (left) and 5 μg/μl spectinomycin (right).

FIG. 4.

(A) Growth of Candida albicans in the presence of 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine. (Left) BYM agar; (right) BYM agar supplemented with N-acetylglucosamine (4 mg/ml). (B) Growth of Erwinia amylovora in the presence of 2-amino-3-(oxirane-2,3-dicarboxamido)-propanoyl-valine. (Left) 5b agar; (right) 5b agar supplemented with N-acetylglucosamine (4 mg/ml).