Characterization of the effects of terminators and introns on recombinant gene expression in the basidiomycete Ceriporiopsis subvermispora

Development of a 2A peptide-based multigene expression system and its application for enhanced production of ganoderic acids in Ganoderma lucidum

Ganoderma has received much attention for its medicinal value, but the manipulation of multiple genes remains a challenge, hindering the genetic engineering of this species for the development of cell factories. Here, we first showed that the presence of an intron is necessary for the efficient expression of the endogenous cDNA of carboxin-resistant gene (cbx) in G. lucidum. Then, the self-cleaving function of 2 A peptide was investigated in G. lucidum by linking cbx cDNA to the codon-optimized hygromycin B-resistant gene (ophph) using the 2A-peptide sequence. The results showed that cbx cDNA and ophph can be successfully expressed in G. lucidum in a bicistronic manner from a single transcript. Moreover, the expression of both genes was not affected by the order within the 2 A cassette. In addition, simultaneous expression of cbx cDNA, ophph, and codon-optimized yellow fluorescent protein gene (opyfp) was conducted for the first time in G. lucidum using the 2 A peptide-based approach. The developed method was successfully applied to express both cDNA of the 3-hydroxy-3-methylglutaryl coenzyme A reductase (hmgr) and squalene epoxidase gene (se) for enhanced production of ganoderic acids (GAs) in G. lucidum. The engineered strain produced the maximum content of GA-Mk, GA-T, GA-S, and GA-Me were 26.56±3.53,39.58±3.75, 16.54±2.16, and 19.1±1.87 μg/100 mg dry weight, respectively. These values were 3.85-, 4.74-, 3.65-, and 3.23-fold higher than those produced by the control strain. The developed method will be useful for the manipulation of complex metabolic or regulatory pathways involving multiple genes in Ganoderma.

CRISPR/Cas9 using a transient transformation system in Ceriporiopsis subvermispora

Ceriporiopsis subvermispora is a white-rot fungus with great potential for industrial and biotechnological applications, such as the pretreatment of lignocellulose in biorefineries, as it decomposes the lignin in the plant cell wall without causing severe cellulose degradation. A genetic transformation system was recently developed; however, gene-targeting experiments to disrupt or modify the gene(s) of interest remain challenging, and this is a bottleneck for further molecular genetic studies and breeding of C. subvermispora. Herein, we report efficient clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9)-assisted gene mutagenesis in this fungus. Two plasmids expressing Cas9 together with a different pyrG-targeting single-guide RNA were separately introduced into the monokaryotic C. subvermispora strain FP-90031-Sp/1, which frequently generated strains that exhibited resistance to 5-fluoroorotic acid and uridine/uracil auxotrophy. Southern blot analyses and genomic polymerase chain reaction followed by DNA sequencing of some mutants revealed that they were pyrG mutants. We also observed that hygromycin resistance of the pyrG mutants was frequently lost after repeated subcultivations, indicating that a maker-free genome editing occurred successfully. It is also suggested that a gene mutation(s) can be introduced via a transient expression of Cas9 and a single-guide RNA; this feature, together with high-frequency gene targeting using the CRISPR/Cas9 system, would be helpful for studies on lignocellulose-degrading systems in C. subvermispora. KEY POINTS: • Efficient plasmid-based CRISPR/Cas9 was established in C. subvermispora. • The mutations can be introduced via a transient expression of Cas9 and sgRNA. • A maker-free CRISPR/Cas9 is established in this fungus.

A promoter assay system using gene targeting in agaricomycetes Pleurotus ostreatus and Coprinopsis cinerea

A novel promoter assay was developed for Agaricomycetes, using a gene-targeting approach, with or without the CRISPR/Cas9 technique. It enables precise evaluation of promoter activity at the original site of the chromosome without random and multiple integrations in conventional transformation experiments.