Identification of Genes Involved in the Synthesis of the Fungal Cell Wall Component Nigeran and Regulation of Its Polymerization in Aspergillus luchuensis

Abstract

Certain Aspergillus and Penicillium spp. produce the fungal cell wall component nigeran, an unbranched D-glucan with alternating α-1,3- and α-1,4-glucoside linkages, under nitrogen starvation. The mechanism underlying nigeran biosynthesis and the physiological role of nigeran in fungal survival are not clear. We used RNA-seq to identify genes involved in nigeran synthesis in the filamentous fungus Aspergillus luchuensis when grown under nitrogen-free conditions. agsB, which encodes a putative α-1,3-glucan synthase, and two adjacent genes ( agtC and gnsA ) were upregulated under conditions of nitrogen starvation. Disruption of agsB in A. luchuensis (Δ agsB ) resulted in the complete loss of nigeran synthesis. Furthermore, overexpression of agsB in an Aspergillus oryzae strain that cannot produce nigeran resulted in nigeran synthesis. These results indicated that agsB encodes a nigeran synthase. Therefore, we have renamed the A . luchuensis agsB as nigeran synthase gene ( nisA ). Nigeran synthesis in an agtC mutant (Δ agtC ) increased to 121%; conversely, that in Δ gnsA and Δ agtCgnsA decreased to 64% and 63%, respectively, compared to that in the wild-type strain. Our results revealed that AgtC and GnsA play an important role in regulating not only the quantity of nigeran but also its polymerization. Collectively, our results demonstrated that nisA ( agsB ) is essential for nigeran synthesis in A . luchuensis, whereas agtC and gnsA contribute to the regulation of nigeran synthesis and its polymerization. This research provides insights into fungal cell wall biosynthesis, specifically the molecular evolution of fungal α-glucan synthase genes and the potential utilization of nigeran as a novel biopolymer. Importance The fungal cell wall is composed mainly of polysaccharides. Under nitrogen-free conditions, some Aspergillus and Penicillium spp. produce significant levels of nigeran, a fungal cell wall polysaccharide composed of alternating α-1,3-/1,4-glucosidic linkages. The mechanisms regulating the biosynthesis and function of nigeran are unknown. Here, we performed RNA sequencing of Aspergillus luchuensis cultured under nitrogen-free or low-nitrogen conditions. A putative α-1,3-glucan synthase gene, whose transcriptional level was upregulated under nitrogen-free conditions, was demonstrated to encode nigeran synthase. Furthermore, two genes encoding an α-glucanotransferase and a hypothetical protein were shown to be involved in controlling nigeran content and molecular weight. This study reveals genes involved in the synthesis of nigeran, a potential biopolymer, and provides a deeper understanding of fungal cell wall biosynthesis.

A Glycosylphosphatidylinositol-Anchored α-Amylase Encoded by amyD Contributes to a Decrease in the Molecular Mass of Cell Wall α-1,3-Glucan in Aspergillus nidulans

“…In the kuro (black) koji mold Aspergillus luchuensis, disruption of agsE (orthologous to A. nidulans agsB) improves the protoplast formation (Tokashiki et al, 2019). Recently Uechi et al revealed that A. luchuensis agsB (no ortholog in A. nidulans) plays a role in nigeran synthesis (Uechi et al, 2021). In A. nidulans, α-1,3-glucan in vegetative hyphae is synthesized mainly by AgsB (Yoshimi et al, 2013;He et al, 2014).…”

“…luchuensis agsB (no ortholog in A . nidulans ) plays a role in nigeran synthesis (Uechi et al, 2021 ). In A .…”

Fungi for the production of new types of biomaterials

“…Furthermore, all found proteins but S. commune (EFI96050.1, EFI95542.1) were predicted by NetGPI v1.1 to possess a GPI anchor at the outmost C-terminus suggesting that they are membrane-anchored like A. nidulans GH13_1 protein AmyD (see above). This however suggests, that these GH13_1 enzymes of the five fungi will possibly contribute to cell-wall linked processes like modification and degradation of self-produced glucans, similar as reported above for AmyD in A. nidulans (van der Kaaij et al, 2007;He et al, 2017;Yoshimi et al, 2017) and homologous proteins of other Aspergili (Uechi et al, 2021). S. commune (EFI96050.1, EFI95542.1) in contrast can be expected to be secreted freely into the environment.…”

Apple Glycosyltransferase MdUGT73AR4 Glycosylates ABA to Regulate Stomatal Movement Involved in Drought Stress

“…It has been shown that the expression of glycosyltransferase genes in various tissues of plants exhibits spatio-temporal specificity [ 36 ]. AtUGT75B1 is expressed in tissues such as germinating seeds, seedlings, root tips, leaves, and flowers, which can be regulated by salt and drought stress, and the level of expression of the gene increases with the prolongation of stress time [ 37 ].…”