Distinct function of Mediator subunits in fungal development, stress response and secondary metabolism in fungal pathogen Fusarium verticillioides

Abstract

Mediator is a conserved, nucleus-localized, multi-subunit complex highly conserved across eukaryotes. It interacts with RNA polymerase II transcription machinery as well as various transcription factors to regulate gene expression. However, systematic characterization of Mediator complex has not be thoroughly performed in filamentous fungi. In current study, our aim was to investigate key biological functions of Mediator subunits in a mycotoxigentic plant pathogen Fusarium verticillioides. While we recognized some level of divergence in subunits constituent, overall structure remained conserved between Saccharomyces cerevisiae and F. verticillioides. We were able to generate 11 Mediator subunit deletion mutants and characterize serveal important phenotypes associated with fungal development, environmental stress, nutrient utilization, virulence as well as secondary metabolism. While each Mediator subunit deletion mutant showed deficiencies in at least three of the phenotypes tested, it is important to note that different modules or subunits showed unique regulatory role in different cellular processes. Significantly, the deletion of FvMed1 led to increased FB1 production, and we were able to confirm that FvMed1 is transported from the nucleus to the cytoplasm under fumonisin-producing conditions. Taken together, our study characterized the different biological functions of Mediator subunits in plant pathogen F. verticillioides and the possibility of select subunits unique cytoplasmic function independent of the core Mediator. IMPORTANCEMediator is a highly conserved component of RNA polymerase II pre-initiation complex (PIC) in eukaryotes. Studies in yeast have demonstrated the important roles of Mediator subunits, but we lack deeper understanding in filamentous fungi. Here, we selected Fusarium verticillioides, a major fungal pathogen of maize worldwide, to study transcriptional profile followed by mutational analyses of all Mediator genes. Each Mediator subunit deletion mutant showed some shared deficiencies but it is also important that different modules or subunits showed unique regulatory role in different cellular processes. Significantly, FvMed1 deletion mutant showed elevated levels of FB1 production, and we were able to confirm that FvMed1 translocates from the nucleus to cytoplasmic organelles under fumonisin-producing conditions. This is a first study demonstrating that a Mediator subunit regulate cellular functions while not directly associating with PIC in filamentous fungi.