Transcription Factors BbPacC and Bbmsn2 Jointly Regulate Oosporein Production in Beauveria bassiana

Abstract

ABSTRACT The entomopathogenic fungus Beauveria bassiana can produce the secondary metabolite oosporein under alkaline conditions or in fungus-killed cadavers. However, the regulatory mechanism of oosporein synthesis is not fully understood. In thisstudy, we found that the pH signaling transcription factor BbPacC is involved in the regulation of oosporein production. Overexpression of BbPacC promotes oosporein production in B. bassiana at pH 6.0 or under alkaline conditions (pH 8.0), but deletion of this gene abolished oosporein production. Under acidic conditions (pH 4.0), no oosporein production was observed in the wild-type and BbPacC overexpression strains. Yeast one-hybrid assays and electrophoretic mobility shift assay (EMSA) confirmed the binding ability of BbPacC with 4 putative PacC-binding sites in the promoter region of BbOpS3, a transcription factor located in the oosporein synthetic gene cluster regulating the expression of oosporein synthetic genes. Overexpression of Bbmsn2, a previously reported negative regulator of oosporein synthesis, in OEPacC or wild-type strains abolished oosporein production in all tested conditions. However, deletion of Bbmsn2 in the BbPacC overexpression strain significantly improved oosporein production even at pH 4.0. These results indicated that BbPacC is a positive regulator of oosporein production and functions jointly with Bbmsn2 to regulate oosporein production in different environments and particularly under alkaline conditions. IMPORTANCE B. bassiana produces the red dibenzoquinone pigment oosporein under certain specific conditions, such as alkaline conditions and fungus-killed cadavers. Ooporein possesses antibiotic and insect immune inhibition activities and plays multiple roles during the infection process of B. bassiana against insect hosts. Several negative regulators involved in oosporein synthesis have been reported; however, we know little about the positive regulators outside the biosynthetic gene cluster. Here, we found that the pH signaling transcription factor BbPacC positively regulates oosporein production by binding to several PacC-binding sites. In addition, our results also indicate that BbPacC jointly acts with the negative regulator Bbmsn2 to regulate oosporein synthesis. Our results provide insight into understanding the regulatory mechanism of oosporein production as well as targets to engineer B. bassiana strains producing high levels of oosporein.

Keywords

• Beauveria bassiana
• oosporein
• regulatory mechanism
• transcription factor