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Transformation of Bacteria by Electroporation Lucy Drury 1. Introduction The use of an electrical field to reversibly permeabilize cells (electroporation) has become a valuable technique for transfer of DNA into both eukaryotic and prokaryotic cells. Many species of bacteria have been successfully electroporated (1) and many strains of E. coli are routinely electrotransformed to efficiencies of lo9 and lOi transformants/pg DNA. Frequencies of transformation can be as high as 80% of the surviving cells and DNA capacities of nearly 10 ~.rgo f transforming DNA/mL are possible (2). The benefit of attaining such high efficiency of transformation is apparent,f or example, in the caseo f plasmid libraries. It is often preferable to construct a library in a plasmid owing to its small size and flexibility. In addition, it is invaluable where the use of a shuttle vector is required for the subsequent transfection of eukaryotic cells. Chemical methods of making cells transformation competent are unable to produce high enough efficiencies to make this kind of library possible. Several commercial machines are available that deliver either a square wave pulse or an exponential pulse. Since most of the published data has been obtained using an exponential waveform, this discussion will be confined to that pulse shape. An exponential pulse is generated by the discharge of a capacitor. The voltage decays over time as a function of the time constant 7. From: Methods In Molecular Biology, Vol. 3 1 Protocols for Gene Anaiysrs Edlted by’ A J Harwood CopyrIght 01994 Humana Press Inc , Totowa, NJ 1 2 Drury R is the resistance in ohms (a), C is the capacitance in Farads, and z is the time constant in seconds. The potential applied across a cell suspension will be experienced by any cell as a function of field strength (E = V/d, where d is the distance between the electrodes) and the length of the cell. A voltage potential develops across the cell membrane; when this exceeds a threshold level the membrane breaks down in localized areas and the cell becomes permeable to exogenous molecules. The permeability produced is reversible provided the magnitude and the duration of the electrical field does not exceed some critical limit, otherwise the cell is irreversibly damaged. Since there is an inverse relationship between field strength and cell size, prokaryotes require a higher field strength for permeabilization than do eukaryotic cells. If the voltage and therefore the field strength is reduced, a longer pulse time is required to obtain the maximum efficiency of transformation, however, this range of compensation is limited (2). Increasing the field strength causes a decrease in cell viability and maximum transformation efficiencies are usually attained when about 30-40% of the cells survive. In this chapter I will describe and discuss the methodology of bacterial electroporation with particular reference to E. coli. 2. Materials 2.1. Making Electrocompetent Bacteria 1, A suitable strain of E. coli: I find MC1061, or its ret- derivative, WMl 100,t ransformw ith theh igheste fficiency. Seer ef. I for others trains. 2. L-Broth: 1% Bacto tryptone, 0.5% Bacto yeast extract, 0.5% NaCl. 3. HEPES: 0.1 m/WH EPES, pH 7.0. This may be replacedb y distilled HzO. 4. Disttlled H,O: Sterilized by autoclaving. 5. 10% Glycerol (v/v): In sterile distilled H20. 2.2. Eh?ctroporation of Competent Bacteria 6. Electroporator: Transformation requires a high voltage electroporation device, such as the Blo-Rad (Richmond, CA) gene pulser apparatus used with the pulse controller, and cuvets with 0.2 cm electrode gap. 7. TE: 10 mA4T ris-HCl, pH 8.0, 1 mJ4 EDTA. 8. SOC: 2% Bacto tryptone, 0.5% Bacto yeast extract, 10 mM NaCl, 2.5 KIM KCl, 10 mM MgS04, 20 mM glucose. Transformation of Bacteria 3 3. Methods 3.1. Making Ebctrocompetent Bacteria 1. Grow an overnight culture of the chosen strain in L-broth or any other suitable rich medium. 2. The next day, inoculate 1 L of L-broth wtth 10 mL of the overnight culture and grow at 37°C with good aeration; the best results are obtained with rapidly growing cells (see Note 1). 3. When culture reaches an ODbO,,o f 0.5-l .O, place on ice. The optimum cell density may vary for each different strain but I have found that usually about 0.5 IS the best. 4. Leave on ice for 15-30 min. 5. Centrifuge the bacteria for 10 min at 4OOOg,, keeping them at 4°C. Remove the supematant and discard. 6. Resuspend the cells in an equal volume of either Hz0 or 0.1 mM HEPES, previously chilled on ice (see Note 2). 7. Spin down cells at 4°C and resuspend in half the volume of ice-cold Hz0 or HEPES. Care must be taken since the cells form a very loose pellet in these low ionic solutions. 8. Harvest at 4°C once again and resuspend in 20 mL of ice-cold 10% glycerol. 9. Harvest for the last time and resuspend in 2-3 mL of 10% glycerol. The final cell concentration should be about 3 x lOlo cells/ml. The cells may be used fresh or frozen on dry ice and stored at -70°C where they will remain competent for about 6 mo. Cells may be frozen and thawed several times with little loss of activity (see Note 3). 3.2. Electroporation of Competent Bacteria 1. Chill the cuvets and the cuvet carriage on ice (see Note 4). 2. Set the apparatus to the 25 ~JF capacitor, 2.5 kV, and set the pulse controller unit to 200 R. 3. Thaw an aliquot of cells on ice, or use freshly made cells. 4. To a cold, 1.5~mL polypropylene tube, add 40 pL of the cell suspension and l-5 & of DNA in Hz0 or a low ionic strength buffer such as TE. Mix well and leave on ice for about 1 mm (see Notes 5-8). There is no advantage in a longer incubation time (see Note 9). 5. Transfer the mixture of cells and DNA to a cold electroporatlon cuvet and tap the suspension down to the bottom. 4 Drury 6. Apply one pulse at the above settings. This should result m a pulse of 2.5 kV/cm with a time constant of 4.8 ms (the field strength will be 12.5 kV/cm). 7. Immediately add 1 mL of SOC medium to the cuvet. Resuspend the cells and remove to a 17 x loo-mm polypropylene tube; incubate the cell suspension at 37°C for 1 h (see Note 10). Shaking the tubes at 225 rpm durmg this mcubation may improve recovery of transformants. 8. Plate out appropriate dilutions on selective agar. There are a number of other problems that may be encountered (see Notes 11-15). 4. Notes 1. To achieve highly electrocompetent E. coli, the cells must be fast growing and harvested at early to mid-log phase. 2. Washing and resuspending the bacteria m solutions of low ionic concentration IS important to avoid arcing m the cuvet owing to conduction at the high voltages required for electroporation. 3. A 10% glycerol solution provides ideal cryoprotection for E. coli cells at -70°C. The cell suspension is frozen by ahquoting and placing m dry ice. Quick freezing in liquid nitrogen may be deleterious (3). Several rounds of careful freeze thawing on ice does not seem to affect the level of the cells competence to a great extent. 4. Because of the high field strength necessary, it is best to perform the electroporation at 0-4”C for most species of bacteria. Electroporation of E. coli performed at room temperature results m a loo-fold drop in efficiency. This may be related to the state of the cell membranes, or may be a result of the additional joule heating that occurs during the pulse (4). 5. Transformation efficiency may be adjusted by changing the cell concentration, Raising cell concentration from 0.8 to 8 x 109/mL increases transformation efficiency by lo- to 20-fold (4). A steady increase in the number of transformants obtained has been found at cell concentrations of up to 2.8 x lO1o/mL using a fixed concentration of DNA (3). 6. Transformmg DNA must be presented to the cells as a solution of low ionic strength. As mentioned m Note 2, high ionic strength solutions cause arcing m the cuvet or a very short pulse time with resulting cell death and loss of sample. Salts, such as CsCl and ammomum acetate, must be kept to 10 mM or less It is advisable to have the DNA dis- solved in TE or H,O. This is particularly relevant after a ligation because the ligation buffer has an iomc concentration too high for use dtrectly in an electroporation. The DNA must be precipitated m ethanol/sodium Transformation of Bacteria 5 acetate (carrier tRNA can be used in the precipitation without affecting the transformation frequency); alternatively the ligation can be diluted l/100 and 5 p,L used for electroporation (5). 7. The concentration of transforming DNA present during an electroporation is directly related to the proportion of cells that are transformed. With E. coli this relationship holds over several orders of magnitude, and at high DNA concentrations (up to 7.5 pg/mL) nearly 80% of the surviving cells are transformed (2). This 1s in contrast to chemically treated competent cells where saturation occurs at DNA concentration loo-fold lower, and where a much smaller fraction of the cells are competent to become transformed (6). For purposes where a high efficiency but a low frequency of transformation is required (for library construction where cotransformants are undesirable) a DNA concentration of less than 10 ng/mL and a cell concentration of less than 3 x lOlo is appropriate. Alternatively, when a high frequency of transformation is required, use l-10 mg/mL, which transforms most of the surviving cells (2). 8. The size and topology of the DNA molecules may affect transformation efficiency. It is reported that plasmids of up to 20 kb transform with the same molar efficiency as plasmids of 3 kb and converting these plasmids to a relaxed form does not affect their transforming activity (2). Larger molecules can be taken up but at much lower efficiencies, for example linear h DNA (48 kb) has a molar transformation efficiency of 0.1% that of small plasmids (2). No direct comparison between E. coli plasmids containing the same origin of replication, promoters, and markers but differing only in size has been published, and in my hands different plasmid constructs transform with different molar efficiencies. Powell et al. (7) have compared the uptake of related plasmids in Streptococcus lactis and observed no clear relationship between size and molar transformation efficiency. 9. There is no evidence for binding of the DNA to the cell surface during the transformation process, and thus increasing the preshock incubation time up to 30 min makes very little difference to the number of resulting transformants (2). In support of this observation, experiments by Calvin et al. (8) show that when cells are mixed with radioactively labeled plasmid, only a small percentage of the label remains bound after two washes. In addition, certain species of bacteria, such as Lac- tobacillus casei, secrete nucleases, so increasing the preshock incubation time may be detrimental (9). 10. Immediately after the pulse E. coli cells are quite fragile and rapid addition of the outgrowth medium greatly enhances their viability and transformation efficiency. Even after 1 mm delay the efficiency drops Drury by 3-5-fold and thrs increases to 20-fold after 10 min (2). Outgrowth is necessary for the cells to express any resistance marker Introduced by the transforming plasmtd and is usually for an hour or approximately two cell divisions. 11. There are a number of causes of arcing in the cuvet. One reason could be that the ionic strength of the DNA solution or the cell suspension is too high. It is important that the DNA is resuspended in TE or HzO. If it is a ligation mixture it must be precipitated with 0.3M Na acetate and 2-3 vol of ethanol or diluted IOO-fold m TE or H20. The same problem can be caused by failure to tap the cell/DNA mixture to the bottom of the cuvet. Another likely cause may be that the cuvets and the chamber were chilled on ice and residual H,O on the surfaces induced an arc. If you are electroporating many samples it is not necessary to chill the carriage between every pulse but it is a good idea to dry the carriage between every few samples since condensation can accumulate and cause arcing. 12. Failure to obtain colonies after transformation will be caused either by problems with the cells or the DNA. It is advisable to make a large quantity of an accurate dilution of a supercoiled plasmid, such as pUC 18, to use as a positive control in all experiments. Use this routmely to check the cells you make. (5 pg supercoiled DNA wtll give about 10,000 transformants if your cells are at efficiencies of lo9 transformants/pg DNA.) If no colonies are obtained from the positive control, ensure that the growth conditions and harvesting of the cells were correct. The most competent cells are made from fast growing cells harvested at early to mid-log phase. Keep all the wash solutions at 4°C and keep the cells cold while harvesting. When making a new strain competent it is best to harvest the cells at a range of densities at an ODea of between 0.4-l .O. We have found the best density usually to be around OS. If the electrocompetent cells were previously stored at -7O”C, ensure that they are still viable. To do this, plate out an appropriate dilution of the cells on a nonselective plate. Should the cells only transform with the control, first check the concentration of your DNA. It may also be possible that the DNA contains toxic contaminants such as phenol or SDS. The viability of the cells after electroporation can be checked by plating a sample on a nonselective plate. A survival of 3040% would be expected using the parameters set out in the methods, but check against an equivalent aliquot of the cells transformed with the control DNA. If the DNA is contami- nated, reprecipitate and wash with 70% ethanol, or use GeneCleanTM to remove unwanted chemicals. Transformation of Bacteria 7 13. If a recently prepared batch of cells, already tested for electrocompetence, gives a reduced transformation efficiency, it is likely to be because of problems with the electroporation. It is important that the cuvets and the carriage are chilled so that the starting temperature of the cells is 0-4OC. It is crucial to add the outgrowth medium (kept at room temperature) as quickly as possible to the cells after electroporation. 14. An unexpectedly high apparent transformation, efficiency may have a number of explanations. The simplest explanation is that the selective plates have exceeded their shelf life. DNA contamination can also be a problem owing to the high competency of the cells. It is important to maintain good sterile techniques and careful use of micropipets to avoid crosscontamination with DNA used in previous experiments. Since electroporation can release plasmid from cells, the effects of contamination with previously transformed bacteria will be greatly heightened, espe- cially if the plasmld is present at a high copy number in the cell. 15. The particular problems outlined above apply to E. coli; problems encountered with other bacterial species could be owing to the charac- teristics of that strain. For example, if the bacterium is encapsulated the entry of the DNA may be impeded, and some species secrete nucleases that could destroy the DNA. Certain types of bacteria may require a longer recovery time or a longer time to express the selective marker. If the size of the cell is unusual, it may require a different field strength. To establish electroporation conditions for a novel species, it is best to consult references concerning similar bacterial types (see ref. I for a list of references), for general parameters from which to further optimize. References 1. Bacterial speciesth at have been transformedb y electroporatlon Bio-Rad Labo- ratories, 1414 Harbor Way South,R ichmond, CA 94804. Bulletin 1631, 1990. 2. Dower, W. J., Miller, J. F , and Ragsdale,C W. (1988) High effiaency transformation of E. coli by high voltage electroporatlon Nucleic Acids Rex 16, 6127-6145. 3. Dower, W. J. (1990) Electroporationo f bacteria:a general approacht o genetic transformation, in Genetic Engineering, vol. 12 (Setlow, J. K , ed.), Plenum, NY, pp. 275-295. 4. Shigekawa, K. and Dower, W. J. (1988) Electroporatron of eukaryotes and prokaryotes: A general approach to the introduction of macromolecules into cells. Biotechniques 6,742-75 1. 5 Willson, T. A. and Gough N. M. (1988) High voltage E. coli electro- transformation with DNA following ligation. Nucleic Acids Res. 16, 11820. 6. Hanahan, D (1985) Techniques for transformation of E. coli, in DNA Cloning, vol. 1 (Rickwood, D. and Hames,B . D., eds.) IRL, Oxford, pp. 109-135. Drury 7. Powell, I. B., Achen, M. G., Hillier, A. J., and Davidson, B. E. (1988) A simple and rapid method for genetIc transformation of Lactic streptococci by electroporation. Appl. Envrronm Microblol. 54,655-660. 8. Calvin, N. M. and Hanawalt P C. (1988) High efficiency transformation of bacterlal cells by electroporatlon. J. Bacterial. 170,2796-2801. 9. Chassy, B. M. and Flickinger, J L. (1987) Transformation of Luctobacillus casei by electroporation FEh4S Mlcrobiol Letts. 44, 173-177. CHAPTER 2 Direct Cloning of Qtll cDNA Inserts Into a Plasmid Vector Matthew L. Poulin and Ing-Ming Chiu 1. Introduction Cloning vectors derived from bacteriophage h are used frequently in the construction of both cDNA and genomic DNA libraries (I). The screening of positive plaques from h libraries is relatively easy with the plaque lifting technique of Benton and Davis (2). However, isolating and subcloning recombinant inserts from the phage clones of interest can be a tedious task. Additionally, if the insert comprises more than one restriction fragment, the smaller fragments may be missed during the subcloning steps. Both polymerase chain reaction (PCR) (3,4) and plasmid rescue using the fl origin of replication, as in the hZAP systems (5), were developed to circumvent this problem. However, these sophisticated procedures may not exist in every molecular cloning laboratory and most of the existing cDNA libraries are constructed in hgt vectors. Here we describe a direct method of cloning inserts from hgt phage into a pBR322 cloning vector. The E. coli strains Y 1088, Y 1089, and Y 1090, commonly used for hgt phage infections, contain an endogenous plasmid known as pMC9 (6). This 6.1-kb plasmid is a pBR322 derivative with the 1.7-kb EcoRI fragment containing the ZacI and ZacZ genes cloned into the unique EcoRI site (7). We showed that the endogenous pMC9 DNA is released during the lysis of infected bacteria and can be copurified with the phage DNA (8,9) (Fig. 1). Thus, following digestion with EcoRI to releaset he phage’s From: Methods m Molecular Wlology, Vol 31 Protocols for Gene Analysis Edited by, A J Harwood CopyrIght 01994 Humana Press Inc , Totowa, NJ 9 10 Poulin and Chiu A B Ml2345 Ml2345 Fig. 1. Purification of recombinant phage DNA from bacterial host Y 1088 and digestion with EC&I. (A) Phage DNA isolated from five plaques (lanes 1-5) containing the 1.8 kb newt bek cDNA insert (arrow head) was digested with EcoRI and electrophoresed on a 1% agarose gel. The 4.4-kb and the 1.7-kb fragments from pMC9 can be seen (arrows). (B) Southern blot of the gel in A using the pMC9 plasmid as a probe. Arrows indicate the hybridization of the 4.4- and 1.7-kb fragments. M, L DNA digested with Hi&III and $X DNA digested with HueIII. Sizes of the markers are in kb. cDNA insert, pMC9 is also digested, releasing its 1.7-kb insert. A ligation can be initiated allowing the direct cloning of the hgtll insert into pBR322. This ligation can result in three different products. 1. The pMC9 itself by either nondigestion or religation of its 1.7-kb EcoRI fragment, 2. The self ligation of the 4.4-kb pBR322, or 3. The hgtl 1 insert ligated into the EcoRI site of the pBR322 vector. By transforming in the presence of isopropyl-P-o-thiogalactoside (IPTG) and Sbromo-4-chloro-3-idolyl P-o-galactoside (X-gal) the first result can be distinguished from the second and third by the ZacZ gene product cleaving the X-gal resulting in a blue colony. The last