Thermostable DNA polymerases with proofreading activity, such as Pfu DNA Polymerase (Cat.# M7741),
Pwo DNA polymerase and Tli DNA Polymerase, generate blunt-ended fragments. Nevertheless, PCR products generated using these polymerases can be modified using the A-tailing procedure outlined in Figure 3 and ligated into the pGEM®-T and pGEM®-T Easy Vectors (5). Using this method, only one insert will be ligated into the vector (as opposed to multiple insertions that can occur with blunt-ended cloning). In addition, with T-vector cloning there is no need to dephosphorylate the vector, and there is a low background of religated vector.
Using this procedure with optimized insert:vector ratios, 55–95% recombinants were obtained when Pfu and Tli DNA Polymerases were used to generate the insert DNA (Table 2). It is critical that the PCR fragments are purified using the Wizard® SV Gel and PCR Clean-Up System (Cat.# A9281) or by direct isolation from a gel by other means. In the absence of purification, the proofreading activity of the Pfu, Pwo and Tli DNA Polymerases will degrade the PCR fragments, or remove the 3´-terminal deoxyadenosine added during tailing or the 3´-terminal deoxythymidine from the vector during the A-tailing reaction or ligation. To optimize cloning efficiency, the amount of DNA in the A-tailing reaction and the ligation volumes must be adjusted depending on the molar yield of the purified PCR product. When molar concentrations are high due to small fragment size and/or good amplification, small volumes of the PCR fragment are needed for the A-tailing and ligation reactions. However, when molar concentration is low due to large fragment size and/or poor amplification, large volumes of the
PCR fragment are needed for the A-tailing and ligation reactions. We have successfully used 1–7μl of purified PCR fragment in A-tailing reactions to optimize the insert:vector ratio. (See Section 3.B for further discussion of optimizing the insert:vector ratio.) Recombinants were identified by blue/white screening, and 70–100% were shown to have the correct size insert by PCR. Few recombinants were observed in control reactions in which the PCR fragment was not tailed. These control results confirm that the majority of the pGEM®-T Easy Vector used contained 3´-terminal deoxythymidine and that, during the A-tailing, Taq DNA Polymerase added a 3´-terminal deoxyadenosine to a significant proportion of the PCR fragments.