Siderophores from the Entomopathogenic Fungus Beauveria bassiana

Authors: Krasnoff, Stuart; Howe, Kevin; Heck, Michelle; DONZELLI, BRUNO - Former ARS Employee

Submitted to: Journal of Natural Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2020
Publication Date: 2/14/2020
Citation: Krasnoff, S., Howe, K.J., Heck, M.L., Donzelli, B. 2020. Siderophores from the Entomopathogenic Fungus Beauveria bassiana. Journal of Natural Products. 83(2):296-304.
DOI: https://doi.org/10.1021/acs.jnatprod.9b00698?ref=pdf

Interpretive Summary: The low solubility of iron in nature, especially in alkaline soils, presents an iron bioavailability challenge that many plants, fungi, and bacteria meet by producing siderophores, small molecules with high binding-affinity and selectivity for iron. Producing organisms secrete siderophores into the environment where they scavenge iron. The siderophore-iron complex can then be recovered by the secreting organism which then internalizes the iron payload by specialized biochemical processes. In pathogenic microbes siderophores can be seen as weapons deployed by invading pathogens in the battle to control the limited iron resources of the hosts, which themselves have evolved highly specialized processes to protect their iron from being hijacked by the invader. Beauveria bassiana is a cosmopolitan insect-pathogenic fungus that infects a wide range of injurious insect species including insect vectors of plant diseases. Thus, this fungus is important because of its potential as a biocontrol agent of insects and several related strains of Beauveria bassiana have been commercialized as biopesticides and are now important components of integrated pest management programs. In light of the key role played by microbial siderophores in pathogenic processes we explored for siderophores in B. bassiana and found an array of compounds, some known and some novel. The major novel siderophore identified, beauverichelin A, has similarities to a common siderophore type seen in other fungi but is modified by the addition of two methylated sugar units and an oxidized nitrogen. These studies advance our knowledge of the types of compounds produced by this biocontrol fungus and sets the stage for studying how these siderophores contribute to the ability of this fungus to successfully infect and kill insects.

Technical Abstract: We report NMR- and MS-based structural characterizations of fifteen compounds (1-15) , including ten new chemical entities (2-4, 6-9, 11-12, and 15), from the entomopathogenic fungus Beauveria bassiana (Balsamo-Crivelli) Vuillemin. All compounds are related to the coprogen family of hydroxamate siderophores. The siderophore mixture of strain ARSEF #2860 was dominated by dimerumic acid (DA) (1), a known compound. This mixture also included two novel compounds, in which N5-mevalonyl-N5-hydroxyornithine (MHO) replaces one (2) or both (3) of the N5-anhydromevalonyl-N5-hydroxy-ornithine (AMHO) units of DA. ARSEF #2860 also produced siderophores and related compounds that have terminal O-glycosyl linkages to a mannopyranose or 4-O-methyl-mannopyranose unit (4-7) . Beauverichelin A (15) , the di-4-O-methyl-mannosyl analogue of metachelin A, was detected in crude extracts of ARSEF #2860, but only in trace amounts. Electrospray mass spectrometric (ESIMS) screening of a panel of B. bassiana strains revealed two, ARSEF #252 and #1955, that produced substantial amounts of beauverichelin A, along with related analogues varying in the number and methylation state of the mannosyl moieties, and in the substitution of MHO for AMHO groups in the terminal residues. Only the di- and tri- hydroxamates (1-7, 15) showed iron-binding activity in the Chrome azurol S (CAS) assay. In solution with FeCl3 all di- and tri-hydroxamates showed strong ESIMS signals consistent with 1:1 ligand:iron complexes, whereas monohydroxamates (8, 9, and 13) produced much weaker signals consistent with 1:1, 2:1, and/or 3:1 ligand:iron complexes. In a quantitative assay DA and beauverichelin A both showed slightly stronger iron binding than the bacterial siderophore desferrioxamine B.