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Abstract
Cell walls are involved in manifold aspects of fungi maintenance. For several fungi, chitin synthesis, degradation and recycling are essential processes required for cell wall biogenesis; notably, the activity of β-N-acetylglucosaminidases (NAGases) must be present for chitin utilization. For entomopathogenic fungi, such as Metarhizium anisopliae, chitin degradation is also used to breach the host cuticle during infection. In view of the putative role of NAGases as virulence factors, this study explored the transcriptional profile and evolution of putative GH20 NAGases (MaNAG1 and MaNAG2) and GH3 NAGases (MaNAG3 and MaNAG4) identified in M. anisopliae. While MaNAG2 orthologs are conserved in several ascomycetes, MaNAG1 clusters only with Aspergilllus sp. and entomopathogenic fungal species. By contrast, MaNAG3 and MaNAG4 were phylogenetically related with bacterial GH3 NAGases. The transcriptional profiles of M. anisopliae NAGase genes were evaluated in seven culture conditions showing no common regulatory patterns, suggesting that these enzymes may have specific roles during the Metarhizium life cycle. Moreover, the expression of MaNAG3 and MaNAG4 regulated by chitinous substrates is the first evidence of the involvement of putative GH3 NAGases in physiological cell processes in entomopathogens, indicating their potential influence on cell differentiation during the M. anisopliae life cycle.
Highlights
Chitin is the second most abundant polymer on Earth and its recycling from carapaces, cuticles and fungal cell walls impacts on carbon and nitrogen cycles
The chitin polymer is composed of b-1,4-linked N-acetyl-D-glucosamine (GlcNAc) subunits (Beier and Bertilsson, 2013) and its degradation can be driven in two ways: i) chitin can be deacetylated to chitosan by action of chitin deacetylases (EC 3.5.1.41), which yields glucosamine monomers via the enzymatic hydrolysis by chitosanase (EC 3.2.1.132); or ii) by the chitinolytic degradation process generating GlcNAc monomers, which involves the initial hydrolysis of the b-1,4 glycoside bonds by the action of a group of enzymes, including chitinases (EC 3.2.1.14), lytic polysaccharide monooxygenases (LPMOs) and b-N-acetylglucosaminidases (NAGases; EC 3.2.1.52) (Beier and Bertilsson, 2013; Thorat et al, 2017)
All other fungal GH20 NAGase sequences and GH20 conserved domain sequences used as queries resulted in alignments with the same two previously detected contigs
Summary
Chitin is the second most abundant polymer on Earth and its recycling from carapaces, cuticles and fungal cell walls impacts on carbon and nitrogen cycles. As has been shown for the mycopathogenic fungus Trichoderma atroviride, chitin could not be used as a nutrient source if NAGase activity is absent, despite the presence of approximately 30 chitinase genes, emphasizing the importance of these enzymes for the full degradation of the chitin polymer (López-Mondéjar et al, 2009). In this way, the diversity of chitinase genes contrasts with the relatively low number of NAGase genes and their fundamental importance on chitin metabolism
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