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Fungal β-lactamases, what’s up with that?

Scott Gold: USDA-ARS Toxicology and Mycotoxin Research Unit

<div>We are characterizing the role of fungal lactamase encoding genes in the environmental competitiveness of <em>Fusarium verticillioides (Fv)</em>, a fungus commonly infecting maize and producing the fumonisin mycotoxins. The role of bacterial β-lactamases as a mechanism of resistance to β-lactam antibiotics (such as penicillin) is well documented. Large families of genes encoding metallo- (PFAM PF00753) and serine-based (PF00144) β-lactamases exist in some fungi. Interestingly, soil borne fungal genomes are highly enriched in lactamase encoding genes. For example, <em>Fv </em>has 46 genes. In contrast, the environmentally isolated foliar obligate plant pathogens, such as <em>Puccinia graminis</em>, <em>Blumeria gramini</em>s, and <em>Taphrina deformans</em> genomes encode only 5, 2, and 5 lactamases. Our central hypothesis is that in the microbially diverse soil environment, fungi are exposed to numerous xenobiotics that they must detoxify. We currently know the function of only one of the <em>Fv </em>lactamase genes. <em>MBL1</em> (FVEG_08291) is required for degradation of the plant-derived benzoxazinone phytoanticipins like BOA. <em>MBL1 </em>expression is highly and specifically induced upon exposure to BOA. This then serves as our paradigm: lactamase genes induced by specific xenobiotics are predicted to confer resistance to the inducing xenobiotic. We are taking a broad approach to identify interactions that induce specific <em>Fv</em> lactamases. Assays for expression of specific lactamases are underway and include interactions with soil isolated <em>Streptomyces</em> species and with <em>Sarocladium zeae</em>, a fungal co-inhabitant with <em>Fv</em> in maize seed. We have generated a set of single lactamase gene deletion mutants to test the role of individual genes in conferring resistance to specific stimuli.</div>