Location: Emerging Pests and Pathogens ResearchTitle: Metachelins, mannosylated and N-oxidized coprogen-type siderophores from Metarhizium robertsii Author
|Keresztes, Ivan - Cornell University - New York|
|Donzelli, Bruno G. - Cornell University - New York|
Submitted to: Journal of Natural Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2014
Publication Date: 7/3/2014
Citation: Krasnoff, S., Keresztes, I., Donzelli, B.G., Gibson, D.M. 2014. Metachelins, mannosylated and N-oxidized coprogen-type siderophores from Metarhizium robertsii. Journal of Natural Products. 77(7):1685-1692.
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 bind iron. The siderophore-iron complex can then be recovered by the producing organism which then internalizes the iron payload by specialized biochemical processes. In pathogenic processes 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. Metarhizium robertsii is an insect-pathogenic fungus that is widespread geographically and infects a wide range of injurious insect species. Thus, this fungus is important because of its potential as a biocontrol agent of insects and several related species of Metarhizium 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 M. robertsii and found an array of compounds, some known and some novel. The major siderophore identified, metachelin A, has similarities to a common siderophore type seen in other fungi but is modified by the unprecedented addition of two 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: Under iron-depleted culture conditions, the entomopathogenic fungus Metarhizium robertsii (Bischoff, Humber, and Rehner) (= M. anisopliae) produces a complex of extracellular siderophores including novel O-glycosylated and/or N-oxidized coprogen-type compounds as well as the known fungal siderophores, Na-dimethyl coprogen (NADC) and dimerumic acid (DA). Metachelin A (1), the most abundant component in the M. robertsii siderophore mixture, was characterized as a 1094 Da analogue of NADC that is O-glycosylated by ß-mannose at both terminal hydroxyl groups and N-oxidized at the dimethylated a-nitrogen. The mixture also contained a 1078 Da analogue, metachelin B (2), which lacks the N-oxide modification. Also detected were the aglycone of 1, i.e., the N-oxide of NADC (3), and the monomannoside of DA (6). N-oxide and O-glycosyl substituents are unprecedented among microbial siderophores. At high ESIMS source energy and at room temperature in DMSO 1 underwent Cope elimination resulting in loss of the Na dimethyl group and dehydration of the a-ß bond. High resolution ESIMS data confirmed that all tri- and di-hydroxamate siderophores (1-6) complex with trivalent Fe, Al, and Ga. In a chrome azurol S (CAS) assay, all of the M. robertsii siderophores showed iron-binding activity roughly equivalent to that of desferrioxamine B.