Velvet-mediated repression of β-glucan synthesis in Aspergillus nidulans spores

Beta-glucans are a heterologous group of fibrous glucose polymers that are a major constituent of cell walls in Ascomycetes and Basidiomycetes fungi. Synthesis of beta (1,3)- and (1,6)-glucans is coordinated with fungal cell growth and development, thus, is under tight genetic regulation. Here, we report that beta-glucan synthesis in both asexual and sexual spores is turned off by the NF-kB like fungal regulators VosA and VelB in Aspergillus nidulans. Our genetic and genomic analyses have revealed that both VosA and VelB are necessary for proper down-regulation of cell wall biosynthetic genes including those associated with beta-glucan synthesis in both types of spores. The deletion of vosA or velB results in elevated accumulation of beta-glucan in asexual spores. Double mutant analyses indicate that VosA and VelB play an inter-dependent role in repressing beta-glucan synthesis in asexual spores. In vivo chromatin immuno-precipitation analysis shows that both VelB and VosA bind to the promoter region of the beta-glucan synthase gene fksA in asexual spores. Similarly, VosA is required for proper repression of beta-glucan synthesis in sexual spores. In summary, the VosA-VelB hetero-complex is a key regulatory unit tightly controlling proper levels of beta-glucan synthesis in asexual and sexual spores.