Transformation ofSchizophyllum commune: An analysis of specific properties

Several properties of transformation in the basidiomycete,Schizophyllum commune, were examined. The transformation efficiency of protoplasts made from germinating basidiospores is dependent upon the length of time that the spores are incubated under conditions that promote germination. Protoplasts prepared from ungerminated spores transform at least 10 times more efficiently than protoplasts prepared from germlings (25 μm in length) or from mycelium. Transformation frequencies of 1000 transformants/μg of control plasmid DNA and 10⁷ protoplasts are sufficient for obtaining transformants with 2 × 10⁷ protoplasts and 10 μg of bank DNA from a genomic plasmid library. The probability of cotransforming with two plasmids is dependent on the DNA concentrations of each; concentrations can be adjusted to yield nearly 100% cotrasformants. The presence of a nonselected plasmid in the reaction mix improves the transformation frequency of a selected marker carried on another plasmid; this is not true if linear fragments ofSchizophyllum genomic DNA are used as the nonselected DNA. Transformation of aSchizophyllum protoplast does not require its fusion to another protoplast.     

Efficient Transformation of Oil Palm Protoplasts by PEG-Mediated Transfection and DNA Microinjection

Background

Genetic engineering remains a major challenge in oil palm (Elaeis guineensis) because particle bombardment and Agrobacterium-mediated transformation are laborious and/or inefficient in this species, often producing chimeric plants and escapes. Protoplasts are beneficial as a starting material for genetic engineering because they are totipotent, and chimeras are avoided by regenerating transgenic plants from single cells. Novel approaches for the transformation of oil palm protoplasts could therefore offer a new and efficient strategy for the development of transgenic oil palm plants.

Methodology/Principal Findings

We recently achieved the regeneration of healthy and fertile oil palms from protoplasts. Therefore, we focused on the development of a reliable PEG-mediated transformation protocol for oil palm protoplasts by establishing and validating optimal heat shock conditions, concentrations of DNA, PEG and magnesium chloride, and the transfection procedure. We also investigated the transformation of oil palm protoplasts by DNA microinjection and successfully regenerated transgenic microcalli expressing green fluorescent protein as a visible marker to determine the efficiency of transformation.

Conclusions/Significance

We have established the first successful protocols for the transformation of oil palm protoplasts by PEG-mediated transfection and DNA microinjection. These novel protocols allow the rapid and efficient generation of non-chimeric transgenic callus and represent a significant milestone in the use of protoplasts as a starting material for the development of genetically-engineered oil palm plants.     

Natural and Electroporation-Mediated Transformation of Methanococcus voltae Protoplasts

The lack of high-efficiency transformation systems has severely impeded genetic research on methanogenic members of the kingdom Archaeobacteria. By using protoplasts of Methanococcus voltae and an integration vector, Mip1, previously shown to impart puromycin resistance, we obtained natural transformation frequencies that were about 80-fold higher (705 transformants per μg of transforming DNA) than that reported with whole cells. Electroporation-mediated transformation of M. voltae protoplasts with covalently closed circular Mip1 DNA was possible, but at lower frequencies of ca. 177 transformants per μg of vector DNA. However, a 380-fold improvement (3,417 transformants per μg of DNA) over the frequency of natural transformation with whole cells was achieved by electroporation of protoplasts with linearized DNA. This general approach, of using protoplasts, should allow the transformation of other methanogens, especially those that may be gently converted to protoplasts as a result of their tendency to lyse in hypotonic solutions.     

Electroporation of Tobacco Protoplasts with Homologous and Nonhomologous Transformation Vectors

A modified protoplast selection/plating technique was used to quantitate and compare the transformation efficiencies of tobacco NTl protoplasts electroporated with plasmid molecules harboring a chimeric nos-neo gene (pMON213) or both this chimeric gene and a region of homology with the host chromosome (pCPI). The latter plasmid was constructed by cloning the pMON213 EcoR I cassette carrying the nos-neo gene into pNtSS233, a pBR322 derivative harboring a 1.2-kbp piece of Nicotiana tabacum DNA comprising the 5'-end of a gene coding for 1,5-ribulose bisphosphate carboxylase-oxygenase (Rubisco). These plasmids were linearized by restriction endonuclease digestion and electroporated into tobacco protoplasts which were subsequently selected on kanamycin-containing medium. Both plasmids displayed the same transformation efficiency, indicating that the presence of a homologous region in the selectable vector had no influence on the rates of transformation.     

Fate of plasmid DNA in transformation of Bacillus subtilis protoplasts

Summary Polyethylene glycol-treated protoplasts of B. subtilis can be transformed by plasmid DNA at very high frequencies (Chang and Cohen 1979). From analysis of plasmid mediated transformation of transformation-deficient mutants it appeared that mutants, reduced in the transformation by plasmid DNA in the competent state, were plasmid transformation-proficient when transformed as protoplasts. By means of CsCl-gradient centrifugation of re-extracted plasmid DNA it could be demonstrated that plasmid DNA enters the protoplasts in the double-stranded form. In addition, sucrose gradient centrifugation of the re-extracted plasmid DNA showed that the entered DNA is predominantly present as covalently closed circular DNA. The efficiency of plasmid transformation in protoplasts was found to be close to one (each plasmid molecule having entered into the protoplasts gives rise to a transformed cell). This is in good agreement with the observation that little, if any, damage is done to this DNA during or after entry into protoplasts.     

Efficient Polyethylene Glycol (PEG) Mediated Transformation of the Moss Physcomitrella patens

A simple and efficient method to transform Physcomitrella pantens protoplasts is described. This method is adapted from protocols for Physocmitrella protonemal protoplast and Arabidopsis mesophyll protoplast transformation¹. Due to its capacity to undergo efficient mitotic homologous recombination, Physcomitrella patens has emerged as an important model system in recent years². This capacity allows high frequencies of gene targeting^3-9, which is not seen in other model plants such as Arabidopsis. To take full advantage of this system, we need an effective and easy method to deliver DNA into moss cells. The most common ways to transform this moss are particle bombardment¹⁰ and PEG-mediated DNA uptake¹¹. Although particle bombardment can produce a high transformation efficiency¹², gene guns are not readily available to many laboratories and the protocol is difficult to standardize. On the other hand, PEG mediated transformation does not require specialized equipments, and can be performed in any laboratory with a sterile hood. Here, we show a simple and highly efficient method for transformation of moss protoplasts. This method can generate more than 120 transient transformants per microgram of DNA, which is an improvement from the most efficient protocol previously reported¹³. Because of its simplicity, efficiency, and reproducibility, this method can be applied to projects requiring large number of transformants as well as for routine transformation.     

Synchronized tobacco protoplasts are efficiently transformed by DNA

Summary A system for the synchronization of tobacco protoplasts was developed using aphidicolin, a mycotoxin which inhibits alpha-like DNA polymerase. Synchronized SR1 tobacco protoplasts were transformed with plasmid-DNA, derived from pLGV neo 11, at different stages of the cell cycle and the frequencies of kanamycin-resistant calli were measured. Compared to unsynchronized protoplasts synchronized cells show a clear increase in transformability provided that the transformation was performed at S- or M-phase. After completion of the M-phase trans-formation efficiencies dropped to the level of unsychronized cells. The efficiency of transformation for synchronized protoplasts was up to 3% of the surviving cells. This is approximately two orders of magnitude higher than the transformation efficiencies for unsynchronized protoplasts.     

An improved method for direct gene transfer and subsequent regeneration ofArabidopsis thaliana Landsbergerecta and two marker lines

A protocol for PEG-mediated transformation of protoplasts is described forA. thaliana ecotype Landsbergerecta and two marker lines derived from it, M4 and M10. The optimal transformation conditions were: 14 μg plasmid DNA and 28 μg carrier DNA per 6 x 10⁵ protoplasts in 15% (w/v) PEG solution. Based on the hygromycin resistance conferred by the transgene, relative transformation frequencies of 2.5–3.2% and absolute transformation frequencies of 1–2 x 10⁻⁴, were obtained. Shoot regeneration frequencies of 40–60% were achieved, and fertile transgenic plants of the three tested lines were obtained. Southern blot hybridizations demonstrated multi-copy integration patterns in most cases. Hygromycin resistance segregation patterns of 3:1 and 15:1 were found, as well as unexpected segregation patterns, suggesting that modifications in gene expression took place and that these can progressively occur over successive generations.     

Optimum Conditions for Electric Pulse-mediated Gene Transfer to Bacillus subtilis Cells

It was found that plasmid DNA (pUB 110) can be introduced into not only protoplasts but also intact cells of Bacillus subtilis by electric field pulses. The transformation of, B. subtilis using protoplasts results in an efficiency of 2.5 × 10⁴ transformants per μg of DNA, with a single pulse of 50 jisec with an initial electric field strength of 7kV/cm. Even transformation of intact B. subtilis cells results in a maximum efficiency of 1.5 × 10³ transformants per μg DNA, with a single pulse of 400 μsec with an initial electric field strength of 16kV/cm. The cell survival of protoplasts and intact cells was approximately 100% and 30%, respectively, under the conditions found to be optimal for the transformation process. Plasmid DNA isolated from pUB 110 containing transformants was indistinguishable from authentic preparations of pBU 110 on gel electrophoretic analysis.     

Transformation of Clostridium acetobutylicum Protoplasts with Bacteriophage DNA

Techniques for the transformation of Clostridium acetobutylicum protoplasts with bacteriophage DNA are described. Transformation required regeneration of protoplasts and a 2-h eclipse period.     

Genetic transformation of Chainia and heat attenuation of its restriction system.

A PEG-mediated transformation system for Chainia (NCL 82-5-1) was developed using a broad host range Streptomyces vector, pIJ702. Protoplasts prepared from Chainia (NCL 82-5-1) were regenerated with 5% efficiency. Transformation of the protoplasts with pIJ702 gave 10-20 transformants/micrograms DNA. The low efficiency of transformation is attributed to a restriction system in Chainia; this could be inhibited by treating the protoplasts at 42 degrees C for 10 min just before transformation. The yield of transformants increased 100-fold when pIJ702 was modified by passage in Chainia. Because the plasmid replicon was functional in Chainia and the modified plasmid was stably maintained, the transformation system should be useful for self-cloning in Chainia NCL 82-5-1 of the many commercially important enzymes this strain is known to produce.     

Protoplast transformation of glutamate-producing bacteria with plasmid DNA

A method for polyethylene glycol-induced protoplast transformation of glutamate-producing bacteria with plasmid DNA was established. Protoplasts were prepared from cells grown in the presence of penicillin by treatment with lysozyme in a hypertonic medium. The concentration of penicillin during growth affected the efficiency of formation, regeneration, and polyethylene glycol-induced DNA uptake of protoplasts. Regeneration of protoplasts was accomplished on a hypertonic agar medium containing sodium succinate and yeast extract. The spectinomycin and streptomycin resistance plasmid pCG4, originally from Corynebacterium glutamicum T250, could transform various glutamate-producing bacteria such as C. glutamicum, Corynebacterium herculis, Brevibacterium flavum, and Microbacterium ammoniaphilum. The plasmid was structurally unchanged and stably maintained in new hosts. The transformation frequency of most competent protoplasts with pCG4 DNA isolated from primary transformants was high (ca. 10(6) transformants per microgram of covalently closed circular DNA) but was still two orders of magnitude below the frequency of transfection with modified DNA of the bacteriophage phi CGI. The difference was ascribed to the involvement of regeneration in transformation.     

Efficient plasmid transformation of the beta-lactam producer Streptomyces clavuligerus.

The conditions for optimal formation and regeneration of protoplasts of Streptomyces clavuligerus were established. The optimal temperature for regeneration of protoplasts and for transformation was 26 degrees C in three different regeneration media. The best efficiency of transformation was obtained with 40% polyethylene glycol 1000. The efficiencies of regeneration and transformation increased greatly when protoplasts were obtained from cultures in the early stationary phase of growth. The number of transformants per assay increased linearly with rising concentrations of protoplasts. However, the number of transformants per protoplast decreased at concentrations of protoplasts above 1.5 X 10(9). The total number of transformants rose linearly at increasing plasmid DNA concentrations, but the number of the transformants per microgram of DNA became constant at concentrations above 1 microgram of DNA. Transformation frequencies as high as 5 X 10(5) transformants per microgram of DNA were obtained when plasmid pIJ702 was isolated from S. clavuligerus but not when isolated from Streptomyces lividans.     

Efficient plasmid transformation of the beta-lactam producer Streptomyces clavuligerus

The conditions for optimal formation and regeneration of protoplasts of Streptomyces clavuligerus were established. The optimal temperature for regeneration of protoplasts and for transformation was 26 degrees C in three different regeneration media. The best efficiency of transformation was obtained with 40% polyethylene glycol 1000. The efficiencies of regeneration and transformation increased greatly when protoplasts were obtained from cultures in the early stationary phase of growth. The number of transformants per assay increased linearly with rising concentrations of protoplasts. However, the number of transformants per protoplast decreased at concentrations of protoplasts above 1.5 X 10(9). The total number of transformants rose linearly at increasing plasmid DNA concentrations, but the number of the transformants per microgram of DNA became constant at concentrations above 1 microgram of DNA. Transformation frequencies as high as 5 X 10(5) transformants per microgram of DNA were obtained when plasmid pIJ702 was isolated from S. clavuligerus but not when isolated from Streptomyces lividans.     

Insertional mutagenesis of Aspergillus fumigatus

We have investigated transformation with heterologous DNA as a method for insertional mutagenesis of Aspergillus fumigatus. Two methods, polyethylene glycol-mediated transformation of protoplasts and electroporation of germinating spores, were used to establish conditions leading to single-copy integration of transforming DNA at different genomic sites. We have assessed the effect of restriction enzyme-mediated integration (REMI) for both methods. Non-REMI protoplast transformation led to integration of multiple copies of transforming DNA in the majority of transformants. Results of REMI with protoplast transformation varied depending on the enzyme used. Low concentrations of several restriction enzymes stimulated transformation, but of ten enzymes investigated only REMI with XhoI and KpnI resulted in single-copy integration of transforming DNA for the majority of transformants. For protoplast transformation with XhoI- or KpnI-based REMI, 50% and 76% of insertions, respectively, were due to integrations at a genomic enzyme site corresponding to the enzyme used for REMI. Electroporation of spores without addition of restriction enzyme resulted in a high transformation efficiency, with up to 67% of transformants containing a single copy of transforming DNA. In contrast to protoplast transformation, electroporation of spores in the presence of a restriction enzyme did not improve transformation efficiency or lead to insertion at genomic restriction sites. Southern analysis indicated that for both protoplast transformation with REMI using KpnI or XhoI and for electroporation of spores without addition of restriction enzymes, transforming DNA inserted at different genomic sites in a high proportion of transformants.     

Transformation using in vivo and in vitro methylation in Streptomyces griseus

Streptomyces griseus does not readily take up foreign DNA isolated from other Streptomyces species or Escherichia coli, presumably due to its unique restriction-modification systems that function as a barrier for interspecific DNA transfer. To efficiently transform S. griseus by avoiding the restriction barriers, we methylated incoming DNA in vivo and in vitro and treated protoplasts with heat prior to transformation. Whereas heat treatment of protoplasts or methylation of the E. coli-Streptomyces shuttle vectors (pXE4 and pKK1443) did not prominently improve the transformation efficiency, HpaII methylation of the vectors from any E. coli strains tested in this study highly increased the transformation efficiency. The highest transformation efficiency was observed when the shuttle vectors were isolated from the dam, hsd strain of E. coli (GM161) and methylated by AluI and HpaII methyltransferases, and the efficiency was approximately the same as that of the vectors from S. griseus. We identified several restriction-modification systems that decrease the transformation efficiency. This research also led us to understand methylation profiles and restriction-modification systems in S. griseus.     

Efficient transformation of filamentous fungus Pleurotus ostreatus using single-strand carrier DNA

The effects of carrier DNAs on the transformation of the basidiomycete Pleurotus ostreatus were analyzed. When lambda phage DNA was added to a transformation mixture containing protoplasts and CbxR vector plasmid, an increased number of drug-resistant transformants was observed on a screening plate containing 2 microg carboxin/ml. The highest efficiency (about 200 transformants/microg vector plasmid) was obtained by the addition of heat-denatured lambda DNA, which gave yields approximately 50-fold higher than the control experiment without a carrier DNA. To our knowledge, this is the first report on enhancement in transformation efficiency of fungal protoplasts by single strand carrier DNA.     

High reliability transformation of the basal fungus Mucor circinelloides by electroporation

Molecular approaches to study the biology of the zygomycete Mucor circinelloides depend mainly on the existence of a polyethylene glycol-based transformation method, which is one of the most efficient in zygomycete fungi. However, the poor reliability and low transformation rates of this method are major obstacles in the molecular study of a number of biological processes. This paper describes an easy and reliable method to transform M. circinelloides protoplasts by electroporation. A high-voltage pulse of 25 μF capacitance, 400 Ω resistance, and 4 kV/cm field strength were seen to be the optimal electrical conditions for delivering DNA into M. circinelloides protoplasts. Under these electrical conditions, successful transformations were carried out with several self-replicative plasmid and strain combinations, producing up to more than 500 transformants per μg DNA. Targeted DNA integration of a transgene (atfA gene of Acinetobacter baylyi) in a particular locus (carRP) was also achieved. This transformation method will considerably facilitate in-depth molecular genetic studies of the biology of this fungus.     

Preparation of leaf mesophyll protoplasts for transient gene expression in Brachypodium distachyon

Transient gene expression systems using protoplasts have been widely used for rapid functional characterization of genes in many plant species. Brachypodium distachyon has recently been employed as a model plant for studies on biofuel grass species and grass crops because of its small genome size, short life-span, and availability of efficient transformation systems. Here, we report the an efficient protocol for the preparation of leaf mesophyll protoplasts from Brachypodium seedlings. We also modified the polyethylene glycol (PEG)-mediated transformation procedure to optimize experimental conditions, such as duration of enzyme digestion, PEG incubation time, and plasmid DNA concentration and size. The green fluorescence protein (GFP)- and ??-glucuronidase (GUS)-coding genes were used as reporters to evaluate the feasibility of this transient expression system. We found that the yield of viable protoplasts was highest after 3 h of enzymatic digestion. Viability of enzyme-digested protoplasts was moderately maintained up to 24 h in Mmg preincubation solution. In addition, the highest transient expression of reporter genes was obtained when protoplasts were transformed with 20 ??g of plasmid DNA and incubated for 16 h. Together with the recent completion of the Brachypodium genome sequence, the Brachypodium transient expression system using leaf mesophyll protoplasts can be widely used for cellular, molecular, and biochemical studies of genes involved in carbon metabolism and signaling pathways mediating intrinsic and environmental cues.     

High frequency transformation of Bacillus subtilis protoplasts by plasmid DNA.

Summary A highly efficient method for transformation of Bacillus subtilis by plasmid DNA is reported. The procedure, which involves polyethylene glycolinduced DNA uptake by protoplasts and subsequent regeneration of the bacterial cell wall, yields up to 80% transformants with an efficiency of 4x10⁷ transformants per g of supercoiled DNA. Plasmids constructed by in vitro ligation or endonuclease-generated fragments of linear plasmid DNA can also transform PEG-treated protoplasts, but at a lower frequency.