Biolistic transformation of Mucor circinelloides

Recent Molecular Tools for the Genetic Manipulation of Highly Industrially Important Mucoromycota Fungi

Mucorales is the largest and most well-studied order of the phylum Mucormycota and is known for its rapid growth rate and various industrial applications. The Mucorales fungi are a fascinating group of filamentous organisms with many uses in research and the industrial and medical fields. They are widely used biotechnological producers of various secondary metabolites and other value-added products. Certain members of Mucorales are extensively used as model organisms for genetic and molecular investigation and have extended our understanding of the metabolisms of other members of this order as well. Compared with other fungal species, our understanding of Mucoralean fungi is still in its infancy, which could be linked to their lack of effective genetic tools. However, recent advancements in molecular tools and approaches, such as the construction of recyclable markers, silencing vectors, and the CRISPR-Cas9-based gene-editing system, have helped us to modify the genomes of these model organisms. Multiple genetic modifications have been shown to generate valuable products on a large scale and helped us to understand the morphogenesis, basic biology, pathogenesis, and host–pathogen interactions of Mucoralean fungi. In this review, we discuss various conventional and modern genetic tools and approaches used for efficient gene modification in industrially important members of Mucorales.

Genetic manipulation of Fonsecaea pedrosoi using particles bombardment and Agrobacterium mediated transformation

Fonsecaea pedrosoi, a melanized fungal pathogen that causes Chromoblastomycosis, a human disease with a worldwide distribution. Biolistic is a widely used technique for direct delivery of genetic material into intact cells by particles bombardment. Another well-established transformation method is Agrobacterium-mediated transformation (ATMT), which involves the transfer of a T-DNA from the bacterium to the target cells. In F. pedrosoi there are no reports of established protocols for genetic transformation, which require optimization of physical and biological parameters. In this work, intact conidia of F. pedrosoi were particle bombarded and subjected to ATMT. In addition, we proposed hygromycin B, nourseothricin and neomycin as dominant selective markers for F. pedrosoi and vectors were constructed. We tested two parameters for biolistic: the distance of the particles to the target cells and time of cells recovery in nonselective medium. The biolistic efficiency was 37 transformants/μg of pFpHYG, and 45 transformants/μg of pAN7.1. Transformants expressing GFP were successfully obtained by biolistic. A co-culture ratio of 10: 1 (bacterium: conidia) and co-incubation time of 72 h yielded the largest number of transformants after ATMT. Southern blot analysis showed the number of foreign DNA insertion into the genome is dependent upon the plasmid used to generate the mutants. This work describes for the first time two efficient methods for genetic modification of Fonsecaea and these results open new avenues to better understand the biology and pathogenicity of the main causal agent of this neglected disease.

Metabolic engineering of Rhizopus oryzae for the production of platform chemicals

Rhizopus oryzae is a filamentous fungus belonging to the Zygomycetes. It is among others known for its ability to produce the sustainable platform chemicals L: -(+)-lactic acid, fumaric acid, and ethanol. During glycolysis, all fermentable carbon sources are metabolized to pyruvate and subsequently distributed over the pathways leading to the formation of these products. These platform chemicals are produced in high yields on a wide range of carbon sources. The yields are in excess of 85 % of the theoretical yield for L: -(+)-lactic acid and ethanol and over 65 % for fumaric acid. The study and optimization of the metabolic pathways involved in the production of these compounds requires well-developed metabolic engineering tools and knowledge of the genetic makeup of this organism. This review focuses on the current metabolic engineering techniques available for R. oryzae and their application on the metabolic pathways of the main fermentation products.

Transformation of Mucor circinelloides with Autoreplicative Vectors Containing Homologous and Heterologous ARS Elements and the Dominant Cbxr Carboxine-Resistance Gene

Mucor circinelloides transformants prototrophic to leucine and resistant to carboxine (Leu(+) Cbx(r)) have been obtained by treatment of protoplasts with plasmid constructs containing homologous leuA gene and adjacent autonomously replicating sequences (ARS) element combined with the Cbx(r)(carboxine-resistance) gene of Ustilago maydis and ARS sequences from this basidiomycete (plasmid pGG37) or from the 2 mu plasmid of Saccharomyces cerevisiae (plasmid pGG43). The presence in the same plasmid molecule of the M. circinelloides leuA gene and adjacent ARS element together with heterologous ARS elements produced an increase in the transformation frequency of about 65-120%. The presence of autoreplicating plasmid molecules in the transformants was demonstrated by mitotic stability experiments, by Southern analysis, and by the rescue of plasmids from transformed bacterial cells.

Development of a system for integrative and stable transformation of the zygomycete Rhizopus oryzae by Agrobacterium-mediated DNA transfer

Two transformation systems, based on the use of CaCl(2)/PEG and Agrobacterium tumefaciens, respectively, were developed for the zygomycete Rhizopus oryzae. Irrespective of the selection marker used, a pyr4 marker derived from R. niveus or a dominant amdS(+) marker from Aspergillus nidulans, and irrespective of the configuration of the transforming DNA (linear or circular), the transformants obtained with the CaCl(2)/PEG transformation method were found to carry multiple copies of tandemly linked vector molecules, which failed to integrate into the genomic DNA. Furthermore, these transformants displayed low mitotic stability. In contrast, transformants obtained by Agrobacterium-mediated transformation were mitotically stable, even under non-selective conditions. Detailed analysis of these transformants revealed that the transforming DNA had integrated into the genome of R. oryzae at a single locus in independently obtained transformants. In addition, truncation of the transforming DNA was observed, resulting in the integration of the R. niveus pyr4 marker gene, but not the second gene located on the transferred DNA. Modification of the transforming DNA, resulting in partial resistance to restriction enzyme digestion, was observed in transformants obtained with the CaCl(2)/PEG transformation method, suggesting that a specific genome defence mechanism may exist in R. oryzae. It is likely that the unique mechanism used by A. tumefaciens to deliver its transferred DNA to its hosts facilitates bypass of the host defence mechanisms, thus allowing the DNA to integrate into the chromosomal genome.