Transcription factor reveals interconnected regulation of carbon source utilization and carbon catabolite repression in an oleaginous yeast
Cells must sense and respond to available nutrients to utilize available resources and establish fungal colonies. Saprophytic microbes harvest carbon from plant biomass, which contain insoluble, complex carbohydrates that must be degraded extracellularly prior to import into the cell. Filamentous fungi can degrade these complex carbohydrates. However, while many single cellular microbes can utilize the building blocks of these insoluble polysaccharides, they frequently are unable to degrade insoluble carbohydrates. Cellobiose is a disaccharide breakdown product of cellulose, the most abundant component of plant biomass. We investigated the genetic regulation of cellobiose utilization in the oleaginous, basidiomycete yeast Rhodotorula (Rhodosporidium) toruloides using a combination of phylogenetic, genetic, and genomic approaches. We tested the closest R. toruloides homolog of the cellulose degradation regulator CLR-2/ClrB for a role in cellobiose utilization. Deletion of this transcription factor, which we named Cbr1, eliminated the ability of cells to grow on media containing cellobiose as the sole carbon source. Although, the role of CLR-2/ClrB is limited to regulating the expression of genes involved in the utilization of cellulose and hemicellulose, Cbr1 plays a role not only in cellobiose utilization, but also utilization of tricarboxylic acid (TCA) cycle intermediates. Transcriptional profiling revealed that the genes activated by Cbr1 included genes encoding beta-glucosidases and transporters. These beta-glucosidases are secreted and are necessary and sufficient for cellobiose utilization, while one of the transporters is required for utilization of the TCA cycle intermediate, citrate. Additionally, Cbr1 inhibits carbon catabolite repression, specifically during utilization of disaccharides, which may have evolved to limit the repression of genes encoding secreted proteins that cleave disaccharides into glucose extracellularly. The co-regulation of cellobiose and TCA cycle utilization with carbon catabolite repression may shed light on alternative methods of the regulation of carbon catabolite repression in fungi. Overall design: mRNA-seq profling of Rhodotorula toruloides wild type and cbr1 deletion strains exposed to cellobiose or succinate as the sole carbon source or media lacking a carbon source for 8h. Each experiment was performed in biological triplicate.