The transcriptional co-activator multiprotein bridging factor 1 from the fungal insect pathogen, Beauveria bassiana, mediates regulation of hyphal morphogenesis, stress tolerance and virulence.

Multiprotein bridging factors (MBFs) are evolutionarily highly conserved cofactors that link TATA-binding protein and the associated basal transcription machinery to transcription factors. The filamentous fungus, Beauveria bassiana, has a multipotential lifestyle capable of growing as a saprophyte, plant endophyte and insect pathogen. Deletion of the single B. bassiana MBF homologue (BbMBF1) affected fungal growth and hyphal morphogenesis, stress response and virulence. Compared with wild type, the ΔBbMBF1 strain displayed increased sensitivity to UV-irradiation and to oxidative, osmotic and heat stress, and decreased virulence in both topical and intrahaemocoel injection bioassays using the greater wax moth, Galleria mellonella larvae. Although only minor radial growth effects were seen for the ΔBbMBF1 strain, aberrant hyphal morphogenesis was observed, which could be rescued by growth in rich broth media. Transcriptional analysis during stress response showed altered gene expression in ΔBbMBF1 during growth under osmotic, oxidative and thermal stress conditions. Genome-wide expression analyses during growth under unstressed and thermal stress conditions revealed global gene expression changes and a set of putative targets for MBF1 mediated gene expression control. Our data indicate that BbMBF1 acts as a key regulatory cofactor controlling stress responses and virulence and that MBF1 dependent and independent pathways control proper hyphal morphogenesis.