Producción y utilización Biotecnológica de nuevas proteínas antifúngicas de hongos filamentosos

Tesis por compendioLos péptidos antimicrobianos (AMP) son una alternativa prometedora para el desarrollo de nuevos antifúngicos que puedan sustituir a los fungicidas usados en agricultura. Sin embargo, el alto coste de la síntesis química y la dificultad para su producción a gran escala han limitado su aplicación. Las proteínas antifúngicas (AFP) son AMP naturales, pequeñas, catiónicas, secretadas y ricas en cisteína con gran potencial para el control de hongos fitopatógenos. Las AFPs se encuentran en hongos filamentosos, son estables y pueden producirse en grandes cantidades. Sin embargo, el papel biológico en su hongo productor no se conoce en profundidad. En esta tesis, se estudió la diversidad de AFPs en genomas de hongos ascomicetos y se propuso una nueva clasificación en tres clases (A, B y C). Penicillium digitatum es el principal patógeno postcosecha de cítricos y codifica solo una AFP en su genoma de clase B (AfpB), mientras que Penicillium expansum, el principal patógeno postcosecha de manzana, codifica una AFP de cada clase (AfpA, AfpB y AfpC). En este trabajo describimos la producción biotecnológica y la caracterización de estas cuatro AFPs. Se ha caracterizado el papel biológico del gen afpB en P. digitatum mediante estudios de expresión génica y generación de mutantes nulos y de expresión constitutiva. Los resultados indicaron que afpB es prescindible para la biología y el ciclo vital del hongo, aunque la expresión del gen afpB bajo el promotor constitutivo gpdA de Aspergillus nidulans es perjudicial para su crecimiento y virulencia. Sorprendentemente, ni la cepa parental ni las cepas constitutivas produjeron cantidades detectables de AfpB a pesar de la alta expresión del gen codificante. El modelado molecular y el diseño racional permitieron predecir la estructura terciaria de AfpB y diseñar péptidos sintéticos para mapear motivos antifúngicos en su secuencia primaria. Confirmamos que los bucles catiónicos L2 y L3 mostraron actividad antifúngica moderada y que pueden actuar sinergísticamente. Con el objetivo de producir AfpB mediante biotecnología, usamos un casete de expresión de AFPs basado en las regiones promotora y terminadora del gen paf de Penicillium chrysogenum, hongo que produce naturalmente grandes cantidades de su propia proteína PAF. Este casete funcionó en P. digitatum y permitió la producción homóloga de AfpB. Los datos también mostraron que las secuencias del péptido señal (SP) y el pro-péptido de la SP-Pro-AfpB no determinan la producción de proteína. También demostramos la estabilidad térmica y la resistencia a la proteólisis de AfpB, y aportamos datos que sugieren que la estructura terciaria no es necesaria para la actividad antifúngica. Similar a lo descrito en P. digitatum, ninguna de las tres AFPs se detectó en los sobrenadantes de cultivo en medio rico de P. expansum. Sin embargo, AfpA se produjo en grandes cantidades en cultivos de medio mínimo de P. expansum. Para completar el repertorio de AFPs, produjimos las tres AFPs de P. expansum (AfpA, AfpB y AfpC) en P. chrysogenum con el casete paf. Las tres proteínas de P. expansum se produjeron, purificaron y caracterizaron con éxito. Ninguna de las AFPs producidas en este trabajo fue citotóxica frente a eritrocitos de mamíferos. AfpA de P. expansum seguida de AfpB de P. digitatum fueron las AFPs más activas contra hongos filamentosos, incluyendo patógenos de plantas y humanos, productores de micotoxinas y sus propios hongos productores, una característica previamente no descrita en las AFPs. Además, la AfpA de P. expansum y la AfpB de P. digitatum protegieron frente a la infección causada por el hongo Botrytis cinerea en plantas de tomate, y AfpA de P. expansum protegió frente a P. digitatum en frutos de naranja. Estos resultados confirman nuestra hipótesis de que las AFPs son buenas candidatas para el desarrollo de nuevos antifúngicos en protección vegetal y conservación postcosecha, pero taEls pèptids antimicrobians (AMP) són una alternativa prometedora per al desenvolupament de nous antifúngics que puguen substituir als fungicides utilitzats en agricultura. No obstant això, l'alt cost de la síntesi química i la dificultat per a la producció biotecnològica a gran escala han limitat la seua aplicació. Les proteïnes antifúngiques (AFP) són AMPs naturals, xicotetes, catiòniques, secretades i riques en cisteína que oferixen un gran potencial per al control de fongs fitopatogens. Les AFPs estan presents de en fongs filamentosos, són molt estables i poden produir-se en grans quantitats. No obstant això, el paper biològic d'estes AFPs en el seu fong productor encara no està clar. En esta tesi es va estudiar la diversitat d'AFPs en genomes de fongs ascomicets i es va proposar una nova classificació en tres clases (A, B i C). Penicillium digitatum, el principal patogen postcollita de cítrics, codifica només una AFP en el seu genoma de classe B (AfpB). Penicillium expansum, el principal patogen postcollita de poma, codifica una AFP de cada classe (AfpA, AfpB i AfpC). En este treball presentem la producció biotecnològica i la caracterització d'estes quatre AFPs. Hem caracteritzat el paper biològic del gen afpB en P. digitatum mitjançant estudis d'expressió gènica i la generació de mutants nuls i d'expressió constitutiva. Els resultats van indicar que afpB és prescindible per a la biologia i el cicle de vida d'este fong, encara que l'expressió del gen afpB davall el promotor constitutiu gpdA d'Aspergillus nidulans és perjudicial per al seu creixement i virulència sobre fruits cítrics. Sorprenentment, ni el cep parental ni els ceps constitutius van produir quantitats detectables d'AfpB malgrat l'alta expressió del gen afpB. El modelatge molecular i el disseny racional van permetre predir l'estructura terciària d'AfpB i dissenyar pèptids sintètics per a identificar motius antifúngics dins de la seqüència primària. Confirmarem que les estructures catiòniques L2 i L3 mostren activitat antifúngica i que poden actuar de forma sinèrgica. Amb l'objectiu de la producció biotecnològica d'AfpB, utilitzarem un casset d'expressió d'AFPs basat en les regions promotora i terminadora del gen paf de Penicillium chrysogenum, el qual produïx naturalment grans quantitats de la seua pròpia proteïna PAF. Este casset va funcionar en P. digitatum i va permetre la producció homòloga d'AfpB. Les dades també van mostrar que les seqüències del pèptid señal (SP) i el propèptid de la SP-Pro-AfpB no determinaren la producció de proteïna. També demostrarem l'extrema estabilitat tèrmica i la resistència proteolítica d'AfpB, i proporcionem dades que suggerixen que l'estructura terciària no és necessària per a l'activitat antifúngica. Semblant a P. digitatum, cap de les tres AFPs es van detectar en els sobrenadants de medi de cultiu ric de P. expansum. Al contrari, AfpA es va produir en grans quantitats en cultius de P. expansum en medi mínim. Per a completar el repertori d'AFPs, vam produir les tres AFPs de P. expansum (AfpA, AfpB i AfpC) en P. chrysogenum mitjançant l'ús del casset paf. Així, les tres proteïnes de P. expansum es van produir, purificar i caracteritzar amb èxit. Cap de les AFPs produïdes en este treball va ser citotóxica front eritròcits de mamífer. AfpA de P. expansum seguida d'AfpB de P. digitatum van ser les AFPs més actives contra fongs filamentosos, incloent patògens de plantes i humans, productors de micotoxines i els seus propis productors, una característica prèviament no descrita per a les AFPs. A més, AfpA de P. expansum i AfpB de P. digitatum van protegir front la infecció causada pel fong Botrytis cinerea en plantes de tomaca, i l'AfpA de P. expansum va protegir front P. digitatum en fruits de taronja. Estos resultats confirmen la nostra hipòtesi anterior de que les AFPs són bones candidates per al desenvolupament d'antifúngics en proteccióAntimicrobial peptides (AMPs) are promising antifungal alternatives to the fungicides used in agriculture. However, the high cost of chemical synthesis and the difficulties of large-scale production have limited their application. Antifungal proteins (AFPs) are a group of natural, small, cationic, secreted, cysteine-rich AMPs that offer a great potential to develop new biomolecules for the control of phytopathogenic fungi. AFPs are naturally present in filamentous fungi, are very stable, and can be produced in large amounts. However, the biological role of these AFPs in their producer fungus is still unclear. In this thesis, we first studied the diversity of AFPs in ascomycetous genomes and proposed a new classification in three different classes (A, B and C). Penicillium digitatum is the main citrus postharvest pathogen and encodes only one AFP from class B in its genome (AfpB), while Penicillium expansum is the main pome postharvest pathogen and encodes one AFP from each class (AfpA, AfpB and AfpC). In this work, we report the identification, efficient biotechnological production and characterization of these four AFPs. We characterized the biological role of the afpB gene in P. digitatum by the study of its gene expression pattern and the generation of null and constitutive expression mutants. Results indicated that afpB is dispensable for the biology and life cycle of this fungus, although expression of the afpB gene under the constitutive Aspergillus nidulans gpdA promoter is detrimental to growth and virulence to citrus. Surprisingly, neither the wild type nor the constitutive strains produced detectable amounts of AfpB in spite of the high afpB gene expression. Molecular modeling and rational design allowed us to predict the AfpB tertiary structure and design synthetic peptides to map antifungal motifs within the AfpB primary sequence. We confirmed that the cationic exposed loops L2 and L3 showed moderate antifungal activity and that they can act synergistically. With the objective of the biotechnological production of AfpB, we used an AFP expression cassette based on the promoter and terminator regions of the well-studied paf gene from Penicillium chrysogenum, which naturally produces high amounts of its own protein PAF. This paf cassette worked efficiently in P. digitatum and allowed the homologous production of AfpB. Data also showed that the signal peptide (SP) and pro-peptide sequences of the translated SP-Pro-AfpB do not determine protein production. We also demonstrated the thermal stability and resistance to proteolytic cleavage of the P. digitatum AfpB, and provided data that suggest that tertiary structure is not required for antifungal activity. Similar to P. digitatum, none of the three AFPs were detected in supernatants of cultures of P. expansum in rich medium. By contrast, AfpA was produced with very high yields in P. expansum cultures in minimal medium. To complete the repertoire of AFPs from P. expansum we produced the three AFPs from P. expansum (AfpA, AfpB and AfpC) in P. chrysogenum with the use of the paf cassette. With this combined approach, the three P. expansum proteins were successfully produced, purified and characterized. None of the four AFPs produced in this work were cytotoxic against mammal erythrocytes. The P. expansum AfpA followed by the P. digitatum AfpB were the most active AFPs against filamentous fungi, including plant and human pathogens, mycotoxin-producer fungi, and their own producers, a feature that had not been previously described for AFPs. Moreover, AfpA from P. expansum and AfpB from P. digitatum protected against fungal infection caused by Botrytis cinerea in tomato plants, and additionally the P. expansum AfpA protected against P. digitatum in orange fruits. These results confirm our previous hypothesis that AFPs are good candidates for the development of antifungals in plant protection and postharvest conservation, but also in clinic or food preservation.Garrigues Cubells, SM. (2018). Producción y utilización Biotecnológica de nuevas proteínas antifúngicas de hongos filamentosos [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/113162TESISCompendi

Caracterización funcional de genes del tipo APF en el hongo fitopatógeno de frutos cítricos Penicillium digitatum

[ES] Penicillium digitatum es el principal patógeno postcosecha de frutos cítricos. Es un hongo ascomiceto necrótrofo que penetra a través de heridas preexistentes e infecta el fruto, afectando su calidad comercial, produciendo su podredumbre y causando graves pérdidas económicas a nivel mundial. Los péptidos antimicrobianos (AMPs), tanto naturales como sintéticos, se consideran una alternativa prometedora frente a la utilización de fungicidas químicos para la protección de los cultivos y la conservación postcosecha de productos vegetales. Entre los AMPs naturales se encuentran las denominadas proteínas antifúngicas (AFPs), pequeñas proteínas catiónicas y ricas en cisteína de origen fúngico. Nuestro grupo de laboratorio ha trabajado en la identificación y caracterización de péptidos antifúngicos sintéticos (PAF) para su aplicación en el ámbito de la protección postcosecha de frutos cítricos. La secuencia del genoma de P.digitatum revela la existencia de genes que podrían codificar proteínas tipo AFP. En este trabajo se han elegido tres de ellos (PdigORF_17907, PDIG_23520, y PDIG_31210) para su caracterización funcional a través de la generación de mutantes nulos mediante el mecanismo de transformación génica mediado por A.tumefaciens (ATMT) y recombinación homóloga. Paralelamente se ha determinado la sensibilidad a péptidos antimicrobianos en diferentes cepas mutantes de P.digitatum, y se ha estudiado la sinergia entre el péptido sintético PAF26 y el péptido PAF112 derivado de la secuencia AFP de P.digitatum PDIG_68840. Los resultados revelan la necesidad de la secuencia completa de PAF112 para ejercer una actividad antimicrobiana sinérgica con PAF26.[EN] Penicillium digitatum is the main citrus postharvest pathogen. It is an ascomycete necrotrophic fungus that penetrates orange fruits through pre-existent wounds, affecting their commercial quality, producing fruit rot and causing economic losses worldwide. Natural and synthetic antimicrobial peptides (AMPs) are considered a potential alternative to chemical fungicides for crop protection. Among natural AMPs, we can find small cationic cysteine-rich proteins called antifungal proteins (AFPs) of fungal origin. In our laboratory we have worked in the identification and characterization of synthetic antifungal peptides (PAF) for their application in the protection of citrus fruits. The P.digitatum recently sequenced genome shows existing genes that could probably codify AFP-like proteins. In this work, we have chosen three of these genes for their functional characterization using the A.tumefaciens mediated transformation mechanism (ATMT) and homologous recombination. On the other hand, the sensitivity to antifungal peptides has been determined in different P.digitatum strains, and the synergy between PAF26 and PAF112, derived from the P.digitatum AFP sequence PDIG_68840, has been identified. The results show the need for the complete sequence of PAF112 to establish an antimicrobial synergetic interaction with PAF26.[CA] Penicillium digitatum és el principal patogen postcollita de fruits cítrics. És un fong ascomicet necròtrof que penetra a través de ferides preexistents i infecta el fruit, afectant a la seua qualitat comercial, produint la seua podridura i causant greus pèrdues econòmiques a nivell mundial. Els pèptids antimicrobians (AMPs), tant naturals com sintètics, es consideren una alternativa prometedora enfront la utilització de fungicides químics per a la protecció dels cultius i la conservació postcollita de productes vegetals. Entre els AMPs naturals es troben les denominades proteïnes antifúngiques (AFPs), unes xicotetes proteïnes catiòniques i riques en cisteïna d'origen fúngic. El nostre grup de laboratori ha treballat en la identificació i caracterització de pèptids antifúngics sintètics (PAF) per a la seua aplicació en l'àmbit de la protecció de fruits cítrics. La seqüència del genoma de P.digitatum revela l'existència de gens que podrien codificar proteïnes tipus AFP. En este treball s'han triat tres d'ells (PdigORF_17907, PDIG23520 i PDIG31210) per a la seua caracterització funcional a través de la generació de mutants nuls per mitjà del mecanisme de transformació gènica mediat per A.tumefaciens (ATMT) i recombinació homòloga. Paral•lelament s'ha determinat la sensibilitat a pèptids antimicrobians en diferents ceps mutants de P.digitatum, i s'ha caracteritzat la sinergia entre l'hexapèptid sintètic PAF26 i el pèptid PAF112 derivat de la seqüència AFP de P.digitatum PDIG_68840. Els resultats revelen la necessitat de la seqüència completa de PAF112 per a exercir una activitat inhibitòria sinèrgica amb PAF26.Garrigues Cubells, SM. (2014). Caracterización funcional de genes del tipo APF en el hongo fitopatógeno de frutos cítricos Penicillium digitatum .http://hdl.handle.net/10251/57212.Archivo delegad

Mapping and Identification of Antifungal Peptides in the Putative Antifungal Protein AfpB from the Filamentous Fungus Penicillium digitatum

Antifungal proteins (AFPs) from Ascomycetes are small cysteine-rich proteins that are abundantly secreted and show antifungal activity against non-producer fungi. A gene coding for a class B AFP (AfpB) was previously identified in the genome of the plant pathogen Penicillium digitatum. However, previous attempts to detect the AfpB protein were not successful despite the high expression of the corresponding afpB gene. In this work, the structure of the putative AfpB was modeled. Based on this model, four synthetic cysteine-containing peptides, PAF109, PAF112, PAF118, and PAF119, were designed and their antimicrobial activity was tested and characterized. PAF109 that corresponds to the gamma-core motif present in defensin-like antimicrobial proteins did not show antimicrobial activity. On the contrary, PAF112 and PAF118, which are cationic peptides derived from two surface-exposed loops in AfpB, showed moderate antifungal activity against P. digitatum and other filamentous fungi. It was also confirmed that cyclization through a disulfide bridge prevented peptide degradation. PAF116, which is a peptide analogous to PAF112 but derived from the Penicillium chrysogenum antifungal protein PAF, showed activity against P. digitatum similar to PAF112, but was less active than the native PAF protein. The two AfpB-derived antifungal peptides PAF112 and PAF118 showed positive synergistic interaction when combined against P. digitatum. Furthermore, the synthetic hexapeptide PAF26 previously described in our laboratory also exhibited synergistic interaction with the peptides PAF112, PAF118, and PAF116, as well as with the PAF protein. This study is an important contribution to the mapping of antifungal motifs within the AfpB and other AFPs, and opens up new strategies for the rational design and application of antifungal peptides and proteins

Heterologous protein production in filamentous fungi

Abstract: Filamentous fungi are able to produce a wide range of valuable proteins and enzymes for many industrial applications. Recent advances in fungal genomics and experimental technologies are rapidly changing the approaches for the development and use of filamentous fungi as hosts for the production of both homologous and heterologous proteins. In this review, we highlight the benefits and challenges of using filamentous fungi for the production of heterologous proteins. We review various techniques commonly employed to improve the heterologous protein production in filamentous fungi, such as strong and inducible promoters, codon optimization, more efficient signal peptides for secretion, carrier proteins, engineering of glycosylation sites, regulation of the unfolded protein response and endoplasmic reticulum associated protein degradation, optimization of the intracellular transport process, regulation of unconventional protein secretion, and construction of protease-deficient strains. Key points: • This review updates the knowledge on heterologous protein production in filamentous fungi. • Several fungal cell factories and potential candidates are discussed. • Insights into improving heterologous gene expression are given

Strategies for the Development of Industrial Fungal Producing Strains

The use of microorganisms in industry has enabled the (over)production of various compounds (e.g., primary and secondary metabolites, proteins and enzymes) that are relevant for the production of antibiotics, food, beverages, cosmetics, chemicals and biofuels, among others. Industrial strains are commonly obtained by conventional (non-GMO) strain improvement strategies and random screening and selection. However, recombinant DNA technology has made it possible to improve microbial strains by adding, deleting or modifying specific genes. Techniques such as genetic engineering and genome editing are contributing to the development of industrial production strains. Nevertheless, there is still significant room for further strain improvement. In this review, we will focus on classical and recent methods, tools and technologies used for the development of fungal production strains with the potential to be applied at an industrial scale. Additionally, the use of functional genomics, transcriptomics, proteomics and metabolomics together with the implementation of genetic manipulation techniques and expression tools will be discussed

Efficient production and characterization of the novel and highly active antifungal protein AfpB from Penicillium digitatum

Filamentous fungi encode distinct antifungal proteins (AFPs) that offer great potential to develop new antifungals. Fungi are considered immune to their own AFPs as occurs in Penicillium chrysogenum, the producer of the well-known PAF. The Penicillium digitatum genome encodes only one afp gene (afpB), and the corresponding protein (AfpB) belongs to the class B phylogenetic cluster. Previous attempts to detect AfpB were not successful. In this work, immunodetection confirmed the absence of AfpB accumulation in wild type and previous recombinant constitutive P. digitatum strains. Biotechnological production and secretion of AfpB were achieved in P. digitatum with the use of a P. chrysogenum-based expression cassette and in the yeast Pichia pastoris with the α-factor signal peptide. Both strategies allowed proper protein folding, efficient production and single-step purification of AfpB from culture supernatants. AfpB showed antifungal activity higher than the P. chrysogenum PAF against the majority of the fungi tested, especially against Penicillium species and including P. digitatum, which was highly sensitive to the self-AfpB. Spectroscopic data suggest that native folding is not required for activity. AfpB also showed notable ability to withstand protease and thermal degradation and no haemolytic activity, making AfpB a promising candidate for the control of pathogenic fungi

CRISPR/Cas9 facilitates rapid generation of constitutive forms of transcription factors in Aspergillus niger through specific on-site genomic mutations resulting in increased saccharification of plant biomass

The CRISPR/Cas9 system has been successfully applied for gene editing in filamentous fungi. Previous studies reported that single stranded oligonucleotides can be used as repair templates to induce point mutations in some filamentous fungi belonging to genus Aspergillus. In Aspergillus niger, extensive research has been performed on regulation of plant biomass degradation, addressing transcription factors such as XlnR or GaaR, involved in (hemi-)cellulose and pectin utilization, respectively. Single nucleotide mutations leading to constitutively active forms of XlnR and GaaR have been previously reported. However, the mutations were performed by the introduction of versions obtained through site-directed or UV-mutagenesis into the genome. Here we report a more time- and cost-efficient approach to obtaining constitutively active versions by application of the CRISPR/Cas9 system to generate the desired mutation on-site in the A. niger genome. This was also achieved using only 60-mer single stranded oligonucleotides, shorter than the previously reported 90-mer strands. In this study, we show that CRISPR/Cas9 can also be used to efficiently change functional properties of the proteins encoded by the target gene by on-site genomic mutations in A. niger. The obtained strains with constitutively active XlnR and GaaR versions resulted in increased production of plant biomass degrading enzymes and improved release of D-xylose and L-arabinose from wheat bran, and D-galacturonic acid from sugar beet pulp.Peer reviewe

FungalBraid 2.0: expanding the synthetic biology toolbox for the biotechnological exploitation of filamentous fungi

[EN] Fungal synthetic biology is a rapidly expanding field that aims to optimize the biotechnological exploitation of fungi through the generation of standard, readyto-use genetic elements, and universal syntax and rules for contributory use by the fungal research community. Recently, an increasing number of synthetic biology toolkits have been developed and applied to filamentous fungi, which highlights the relevance of these organisms in the biotechnology field. The FungalBraid (FB) modular cloning platform enables interchangeability of DNA parts with the GoldenBraid (GB) platform, which is designed for plants, and other systems that are compatible with the standard Golden Gate cloning and syntax, and uses binary pCAMBIA-derived vectors to allow Agrobacterium tumefaciensmediated transformation of a wide range of fungal species. In this study, we have expanded the original FB catalog by adding 27 new DNA parts that were functionally validated in vivo. Among these are the resistance selection markers for the antibiotics phleomycin and terbinafine, as well as the uridine-auxotrophic marker pyr4. We also used a normalized luciferase reporter system to validate several promoters, such as PpkiA,P7760,Pef1¿, and PafpB constitutive promoters, and PglaA,PamyB, and PxlnA inducible promoters. Additionally, the recently developed dCas9-regulated GB_SynP synthetic promoter collection for orthogonal CRISPR activation (CRISPRa) in plants has been adapted in fungi through the FB system. In general, the expansion of the FB catalog is of great interest to the scientific community since it increases the number of possible modular and interchangeable DNA assemblies, exponentially increasing the possibilities of studying, developing, and exploiting filamentous fungi.This work was supported by PROMETEO/2018/066 from "Conselleria d'Educacio" (Generalitat Valenciana, Comunitat Valenciana, Spain), grant PID2021-125858OB-100, and the Severo Ochoa Excellence Program CEX 2021-001189-S funded by MCIN/AEI/10.13039/501100011033 and by "ERDF A way of making Europe." EM-G was the recipient of a predoctoral grant FPU18/02019 funded by MCIN/AEI/10.13039/501100011033 and by "ESF Investing in your future." SG holds a Juan de la Cierva Incorporacion grant (IJC 2020-042749-I) funded by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR.Moreno-Giménez, E.; Gandía, M.; Sáez, Z.; Manzanares, P.; Yenush, L.; Orzáez Calatayud, DV.; Marcos, JF.... (2023). FungalBraid 2.0: expanding the synthetic biology toolbox for the biotechnological exploitation of filamentous fungi. Frontiers in Bioengineering and Biotechnology. 11:1-17. https://doi.org/10.3389/fbioe.2023.12228121171

Non-homologous end-joining-deficient filamentous fungal strains mitigate the impact of off-target mutations during the application of CRISPR/Cas9

CRISPR/Cas9 genome editing technology has been implemented in almost all living organisms. Its editing precision appears to be very high and therefore could represent a big change from conventional genetic engineering approaches. However, guide RNA binding to nucleotides similar to the target site could result in undesired off-target mutations. Despite this, evaluating whether mutations occur is rarely performed in genome editing studies. In this study, we generated CRISPR/Cas9-derived filamentous fungal strains and analyzed them for the occurrence of mutations, and to which extent genome stability affects their occurrence. As a test case, we deleted the (hemi-)cellulolytic regulator-encoding gene xlnR in two Aspergillus niger strains: a wild type (WT) and a non-homologous end-joining (NHEJ)-deficient strain ΔkusA. Initial phenotypic analysis suggested a much higher prevalence of mutations in the WT compared to NHEJ-deficient strains, which was confirmed and quantified by whole-genome sequencing analysis. Our results clearly demonstrate that CRISPR/Cas9 applied to an NHEJ-deficient strain is an efficient strategy to avoid unwanted mutations. IMPORTANCE Filamentous fungi are commonly used biofactories for the production of industrially relevant proteins and metabolites. Often, fungal biofactories undergo genetic development (genetic engineering, genome editing, etc.) aimed at improving production yields. In this context, CRISPR/Cas9 has gained much attention as a genome editing strategy due to its simplicity, versatility, and precision. However, despite the high level of accuracy reported for CRISPR/Cas9, in some cases unintentional cleavages in non-targeted loci-known as off-target mutations-could arise. While biosafety should be a central feature of emerging biotechnologies to minimize unintended consequences, few studies quantitatively evaluate the risk of off-target mutations. This study demonstrates that the use of non-homologous end-joining-deficient fungal strains drastically reduces the number of unintended genomic mutations, ensuring that CRISPR/Cas9 can be safely applied for strain development

A Penicillium chrysogenum-based expression system for the production of small, cysteine-rich antifungal proteins for structural and functional analyses

L