Efficient four fragment cloning for the construction of vectors for targeted gene replacement in filamentous fungi
© Frandsen et al; licensee BioMed Central Ltd. 2008
Received: 30 April 2008
Accepted: 01 August 2008
Published: 01 August 2008
The rapid increase in whole genome fungal sequence information allows large scale functional analyses of target genes. Efficient transformation methods to obtain site-directed gene replacement, targeted over-expression by promoter replacement, in-frame epitope tagging or fusion of coding sequences with fluorescent markers such as GFP are essential for this process. Construction of vectors for these experiments depends on the directional cloning of two homologous recombination sequences on each side of a selection marker gene.
Here, we present a USER Friendly cloning based technique that allows single step cloning of the two required homologous recombination sequences into different sites of a recipient vector. The advantages are: A simple experimental design, free choice of target sequence, few procedures and user convenience. The vectors are intented for Agrobacterium tumefaciens and protoplast based transformation technologies. The system has been tested by the construction of vectors for targeted replacement of 17 genes and overexpression of 12 genes in Fusarium graminearum. The results show that four fragment vectors can be constructed in a single cloning step with an average efficiency of 84% for gene replacement and 80% for targeted overexpression.
The new vectors designed for USER Friendly cloning provided a fast reliable method to construct vectors for targeted gene manipulations in fungi.
Filamentous fungi influence modern society primarily as hazardous pathogens and providers of drug leads and enzymes for the biotechnological companies. The wide interest in filamentous fungi has led to the sequencing and annotation of more than 30 fungal genomes, with another 130 on the way http://www.ncbi.nlm.nih.gov/Genomes/. This resource constitutes a huge potential for future advances in the basic understanding and industrial exploitation of fungal biology.
A key limiting factor in future work will be the speed by which site-directed genome modifications, such as gene deletion, promoter replacement, fusion of coding sequences with reporter genes (eg. GFP) or the introduction of epitope tags can be performed and verified. Site directed modifications in filamentous fungi can be carried out by homologous recombination and is typically achieved by introducing a DNA fragment containing two homologous recombination sequences (HRS) flanking a selection marker. The HRS's are identical to the sequences surrounding the target locus in the genome and are typically amplified by PCR. Homologous recombination between the vector DNA and the genome results in a replacement of the target DNA with the selection marker. The length of HRS required to obtain a satisfactory frequency of homologous recombination varies between fungal species. Contrary to Saccharomyces cerevisiae, where 30 bp is sufficient, many filamentous fungi require longer HRS , eg Fusarium graminearum needs 400 bp  1500 bp is reported for Aspergillus niger  and around 1000 bp for Neurospora crassa . The necessity of including sequences in the range of 1 kb  makes restriction enzyme and ligation dependent cloning inefficient. Several laboratories have solved this problem by dividing the replacement constructs into two, a technique known as bipartite gene-targeting or split-marker recombination [6–8]. In this technique, the two HRS's are fused with two thirds of either the 3' or 5'end of the selection marker gene, by fusion-PCR . Targeted gene replacement in the fungus is then achieved by a triple homologous recombination between the two PCR fragments and the genomic target, resulting in the formation of a functional selection marker gene and replacement of the targeted locus. The advantage of this technique is that vector construction can be omitted and the PCR products used directly. The technique also increases the efficiency of homologous integration compared to traditional techniques . However, integration of unspecific products generated during the required fusion PCR reactions may give secondary mutations that are difficult to identify. The technique is typically used in combination with protoplast based transformation, which can be difficult or even impossible in some fungal species. Another example of a vector construction system is the one developed for high-throughput knockout of genes in Neurospora crassa. Recombinational cloning of the two required HRS with a selection marker gene and a vector backbone is carried out in yeast, followed by PCR amplification of the two HRS and selection marker gene . The amplified DNA is then transformed into N. crassa by electroporation, a technique for which no protocols are available for the majority of fungal species.
In the present study, we designed four new vectors, adapted to the USER Friendly cloning technology. Here we show that the technology can be used to generate a large number of targeted gene replacement and overexpression vectors for high throughput functional analysis of fungal genes.
Reagents and enzymes
Restriction enzymes, T4 DNA polymerase, T4 DNA ligase and Calf Intestinal Phosphatase (CIP) and the USER enzyme mix were purchased from New England Biolabs (Ipswich, MA, USA).
Production of amplicons for the USER Friendly cloning reactions were performed with PfuTurbo® Cx Hotstart DNA polymerase (Stratagene, Cedar Creek, TX, US). The gpdA promoter from Aspergillus nidulans was amplified with Phusion DNA polymerase (Finnzymes, Espoo, Finland). Screening of E. coli transformants was performed with Sigma Taq DNA polymerase (Sigma Aldrich). All PCR reactions were performed using an Eppendorf MasterCycler EP, using the temperature gradient mode for optimization. Sequencing was performed at GATC Biotech (Constance, Germany). Oligos for construction of the USER cloning sites (UCS) were purchased as HPLC purified oligos from Invitrogen (Carlsbad, CA, US). Primers for replacement and overexpression of Fusarium graminearum genes were designed based on the F. graminearum genome sequence from the Broad Institute http://www.broad.mit.edu/annotation and annotations at the F. graminearum Genome DataBase (FGDB)  using Vector NTI 10 (Invitrogen). Primers were purchased from Eurofins MWG|operon (Ebersberg, Germany).
If not specified otherwise all enzymes and kits were used as recommended by the manufacturer.
Preparation of DNA
For production of genomic DNA from F. graminearum, the PH-1 wild type strain was grown in liquid YPG medium  for three days at 25°C in darkness with stir at 150 rpm. The gDNA was purified following the procedure described by Malz . Plasmid DNA was purified using the Miniprep and Maxiprep kits from QIAgen (Chatsworth, CA, USA). Digested plasmids and PCR products were purified using the GFX purification kit from GE Healthcare. DNA concentrations were measured using a NanoDrop ND-1000 spectrophotometer (Wilmington, DE, US).
Construction of pAg1-H3E for in locus overexpression
Construction of the USER vectors, pRF-HU2 and pRF-HU2E, for targeted replacement
To construct vectors with two unique UCS's flanking the selection marker genes, oligos [see Additional file 1] U2(LB)up/down, U2(RB)up/down and U2(LB)Eup/Edown were mixed to a final concentration of 10 mM each, heated to 90°C for 5 minutes and then cooled to room temperature for annealing of complementary sequences. The ends of the oligos include overhangs that allow directional cloning into the ApaI/SacI (Left Border) and SpeI/HindIII (Right Border) sites of the pAg1-H3 vector and into the KpnI/XmaI (LB) and SpeI/HindIII (RB) sites of the pAg1-H3E vector.
Similarly, U2(LB)Eup/Edown was ligated into KpnI and XmaI digested pAg1-H3E, resulting in pAg1-HUE(LB). The resulting vector was then digested with SpeI and HindIII, mixed with U2(RB)up/down and ligated, resulting in pRF-HU2E (Figure 5 and 6) [see Additional file 2].
Construction of pRF-HU and pRF-HUE for random integration
For cloning of single PCR amplicons, vectors with only a single UCS were generated. The intermediate vectors (pAg1-HU(LB) and pAg1-HUE(LB)) from the construction of pRF-HU2 and pRF-HU2E, both contain two PacI sites, one in the introduced UCS at the LB and one in the RB Multiple Cloning Site (MCS). To allow USER Friendly cloning of single PCR amplicons it was necessary to remove the PacI site in the RB MCS. Vectors were digested with SmaI and HindIII and the resulting ends were polished, using T4 DNA polymerase, and self ligated using T4 DNA ligase. The resulting vectors, pRF-HU and pRF-HUE contain a single USER cloning site at their LB (Figure 5 and 6) [see Additional file 2].
The USER Friendly cloning reaction and testing of the system
The designed gene specific primers included 9 bp long 2-Deoxyuridine containing overhangs, O1 = 5'-GGTCTTAAU, O2 = 5'-GGCATTAAU, O3 = 5'-GGACTTAAU, O4 = 5'-GGGTTTAAU, A3 = 5'-GGACTTAAU, A4 = 5'-GGGTTTAAU, which ensured directionality in the cloning reactions (Figure 7). The USER Friendly cloning technique does not require purification of the amplicons prior to cloning and the PCR reactions were therefore used directly, with a DNA concentration ranging from 6 – 15 ng/μl.
For cloning of two PCR amplicons the following components were mixed in a 0.2 ml PCR tube: 1 μl USER enzyme mix (1.0 unit), 10 μl of insert no. 1, 10 μl of insert no. 2 and 200 ng of digested and purified vector in a total reaction volume of 25 μl adjusted with MilliQ water [see Additional file 3]. For single fragment cloning the geneX-A3/A4 fragment was mixed with 1 μl USER enzyme mix and 200 ng pRF-HU or pRF-HUE vectors in a reaction volume of 15 μl. The USER reactions were incubated for 20 minutes at 37°C, followed by 20 minutes at 25°C. The entire volume of the USER cloning reactions were used to transform 50 μl of freshly prepared chemical competent E. coli cells (106 cfu./μg DNA), following the transformation guidelines described in . Transformants were selected by adding 25 μg/ml kanamycin to the medium.
The construction of pRF-HU2::PKS1 and pRF-HU2E::PKS1 vectors were performed in triplicate, using different batches of vector, enzyme and PCR amplicons. Negative controls were performed by replacing the PCR amplicons with MilliQ water.
Verification of the inserts and their orientation
The resulting E. coli transformants for the two PKS1 vectors were analysed with the insert specific primer pair to determine the cloning efficiency. All positive transformants were then analysed with primer combinations revealing the orientation of the inserts in the vectors (ex. PKS1-O1/RF-2 and RF-1/PKS1-O4 or RF-3/PKS1-A4). The cloning efficiency was calculated as the number of transformants that contained both inserts in the correct orientation divided by the total number of transformants obtained in the cloning reaction. The size of the vectors was determined by restriction enzyme digestion. The two inserts in the pRF-HU2:PKS1 vector were sequenced using the RF-1 and RF-2 primers and the two inserts in the pRF-HU2E::PKS1 with the RF-2 and RF-3 primers.
For the other 27 vectors (16 × pRF-HU2 and 11 × pRF-HU2E) that were constructed by USER cloning, ten transformants were picked randomly for each construct and screened with the insert specific primer pairs. The cloning efficiency was calculated as the number of transformants that contained both inserts divided by the number of screened transformants. The size of the individual vectors was evaluated by restriction enzyme digestion, to confirm the cloning efficiency.
Results and Discussion
The annotation of the sequenced fungal genomes has revealed a plethora of predicted proteins which do not show homology to any previously characterized proteins. In F. graminearum PH-1 alone, 8091 out of 13938 (58%) putative genes fall into the categories "Conserved hypothetical protein" or "Hypothetical protein" . Functional characterisation of these putative genes and proteins, to determine their importance for cellular processes related to pathology and synthesis of bioactive secondary metabolites, relies on efficient replacement and overexpression strategies. Implementation of the single step vector construction strategy shown in Figure 1 has the potential to accelerate this research.
The strategy is dependent on a four fragment cloning step, which is laborious and has a very low success rate using classical cloning techniques. The Xi-cloning and In-Fusion technologies allow for four fragment cloning, but had a very low success rate in our experiments (unpublished results). The yeast based construction of gene replacement cassettes for transformation of N. crassa, Colot and co-works , also allows for efficient four fragment cloning. However, it is dependent on a final PCR based amplification step which increases the risk of unintended mutations in the HRS. These could affect the expression and functionality of the genes surrounding the target locus and sequencing of the integration site would be required to ensure that no unintended mutations have been introduced. The finding that the proofreading PfuTurbo Cx DNA polymerase  can amplify templates containing uracil made the USER Friendly cloning technology attractive for our purpose. The use of Proofreading DNA polymerase is essential when making targeted genome modifications in fungi, due to the close spacing of fungal genes , which often means that the HRS extends into neighbouring genes or their regulatory sequences. USER Friendly cloning has been used for the directional fusion of multiple PCR amplicons in a single cloning step, but only into a single site of the recipient vector . To test whether the USER Friendly cloning technique would allow the simultaneous cloning of two PCR amplicons into different sites of a recipient vector, vectors with two UCS's were needed. For this purpose we designed two new 26 bp long UCS's with PacI and Nt.Bbv.CI sites. These were cloned into the ATMT vectors pAg1-H3 and pAg1-H3E, resulting in pRF-HU, pRF-HU2, pRF-HUE and pRF-HU2E (Figure 5), according to the cloning strategy shown in Figure 4. The 26 bp long UCS's are significantly shorter than those utilized in the commercial USER Friendly cloning kit  and those reported in . This is an advantage if the vectors have to be modified by the addition of reporter genes or change of the selection marker gene, as shorter 5' overhangs on the primers will be required. Digestion of the new vectors, containing two UCS's, with PacI and Nt.Bbv.CI results in two fragments with four unique 9 bp long single stranded 3' overhangs, each consisting of seven constant base pairs and two variable bases (Figure 2C). The two variable bases in each overhang provide directionality in the cloning reaction, thereby ensuring correct assembly of the DNA fragments during cloning.
Testing of USER Friendly cloning for single step four fragment cloning
No. of correct colonies/total no. of colonies
The construction of the vectors, by four fragment cloning, for targeted replacement and in locus overexpression of PKS1 was performed in triplicate and the resulting transformants were screened by PCR. Construction of the targeted replacement vector pRF-HU2::PKS1 resulted in an average of 56.3 colonies of which 85.2% tested positive for the two inserts (Table 1). Construction of the overexpression vector pRF-HU2E::PKS1 resulted in an average of 47.3 colonies of which 82.4% were correct. All transformants that tested positive for the inserts also displayed the desired orientation in the vector. The negative controls all resulted in zero colonies. Construction of the seventeen different vectors, including PKS1, for targeted gene replacement (pRF-HU2) in F. graminearum resulted in an average cloning efficiency of 84.1% (+/- 5%) determined by analyses of ten transformants for each of the constructs. The twelve vectors constructed for in locus overexpression (pRF-HU2E vectors), including PKS1, resulted in an average cloning efficiency of 80.0% (+/- 6%) determined by screening ten transformants for each construct. Based on these results, fusion of two PCR amplicons with two vector fragments by USER Friendly cloning proved to be very efficient as also suggested by the successful fusion of multiple PCR amplicons by Geu-Flores and co-workers .
The presence of the same UCS's in the four vectors allow the reuse of primers for different constructs, e.g. replacement and over-expression, thereby reducing the number of primers that are required for the full analysis of a given target gene. This was exploited for the test vectors where the geneX-O1/O2 product is reused for both replacement and overexpression vectors. The designed O1/O2 primer pair amplifies the promoter sequence of the respective target genes, the O3/O4 primers amplifies the 5' end of the coding sequences including the start codon, and the A3/A4 primers amplifies the terminator region of the respective genes (Figure 7A). For construction of targeted gene replacement vectors, pRF-HU2 was mixed with geneX-O1/O2 and geneX-A3/A4. These are required to give homologous recombination with the fungal genome resulting in the replacement of the coding sequence of the target gene (Figure 7B). Vectors for in locus over-expression (promoter insertion) were constructed by mixing pRF-HU2E with geneX-O1/O2 and geneX-O3/O4, so that homologous recombination with the target locus inserts the A. nidulans gdpA promoter in front of the start codon of the target gene (Figure 7C).
An additional advantage of the single step construction strategy is that all vector constructs can utilize the same tested vector preparation, which is not the case for the classical two step cloning strategy where cloning of the second insert is dependent on the preparation and digestion of the vector construction in the first cloning step. Besides minimizing the time expenditure for vector construction, the four fragment cloning offers increased flexibility in the choice of selection marker, since the digested vector-backbone can be purified and the hph cassette (Figure 7B and 7C) replaced with other antibiotic selection markers as zeocin or geneticin. This is of high relevance in repeated transformation of hygromycin resistant mutants. Our average vector construction time with the USER Friendly cloning system is three to four days, compared to 14 days with Xi-cloning and 20 days with classical cloning techniques. Furthermore the single step construction strategy only requires a single verification process, whereas two are required for the classical techniques. The constructed vectors are compatible with both A. tumefaciens mediated transformation and protoplast based transformation of fungi.
The developed USER Friendly cloning system combined with A. tumefaciens mediated transformation, PCR based screening methods [28, 15] and quantitative Real-Time PCR to determine the gene copy number  can be perfected to give a realistic high throughput system for large scale functional studies of genes in filamentous fungi.
The constructed pRF-HU2 and pRF-HU2E vectors allow for the simultaneous directional cloning of two inserts in a four fragment assembly with an efficiency of 84.1% and 80.0%, respectively. These results show that the single step four fragment USER Friendly cloning method provides a good alternative to existing vector construction techniques. Vector construction for targeted replacement of genes is reduced to design of two primer pairs, which will permit automation of the experimental design as required for high-throughput knockout projects . The promoters that drive the hygromycin resistance gene in the designed vectors are probably inefficient in basidiomycotes but favourite vectors can easily be converted to USER compatible vectors simply by introducing the two new UCS's into their MCS's.
Agrobacterium tumefaciens mediated transformation
left border of T-DNA (transfer DNA)
multiple cloning site
polymerase chain reaction
- PKS :
polyketide synthase encoding gene
right border of T-DNA (transfer DNA)
USER cloning site
Uracil Specific Excision Reaction.
We are grateful to Hussam Hassan Nour El Din Auis and Klaus S. Albertsen for introducing us to the USER Friendly cloning technique and for valuable suggestions. We would also like to thank technician Pia Petersen for her excellent work with the vector construction. The research was funded by 'The Danish Ministry of Food, Agriculture and Fisheries', the 'PhD Research School for Biotechnology' (FOBI) and the 'Faculty of Life Sciences' at the 'University of Copenhagen', Denmark.
Vector sequences and protocols can be found as on the BMC molecular biology web-page [see Additional file 2] [see Additional file 3] to the article or on http://www.RasmusFrandsen.dk. Vectors will be shared upon request.
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