Team:Wisconsin-Madison/

Expression and Purification of Enzymes

Gibson Assembly

The Gibson Assembly utilizes three enzymes: (1) a thermostable DNA ligase; (2) a 5’-exonuclease; and, (3) a thermostable DNA polymerase, in an isothermal reaction to assemble together DNA fragments containing 20-40 base pair overlaps. The reaction proceeds as shown in figure 1: First, the exonuclease chews back the 5' ends of the overlapping pieces of DNA, allowing them to anneal. The amount of exonuclease must be finely tuned so that long enough sticky ends are created for the overlaps to anneal before the exonuclease becomes heat inactivated, but not a large enough amount of exonuclease that the DNA is destroyed. The polymerase then fills in the gaps created by the exonuclease until the ligase can join the strand, creating a single, unscarred, double-stranded piece.

Figure 1. Gibson, D.G. et al. Nat. Methods 343-345 (2009)

The Gibson Assembly is extremely useful for cloning. As the figure below shows, a gene can be designed to overlap a backbone plasmid. PCR can then be used to create the overlapping insert and backbone pieces, which can then be Gibson assembled.

Because of its simplicity and versatility, it is a very common cloning technique, however, the cost of the enzymes can be prohibitive for teams with small budgets. Therefore, we have provided an expression vector for T5 exonuclease and taq ligase, as well as a protocol for their expression and purification.

Purification of Enzymes

Following the completion of purification, a number of tests were carried out to verify the legitimacy of the protocol. Firstly, the eluted protein fraction from the Ni-NTA columns were tested against an uninduced control to ensure that a protein of interest had in fact been expressed and purified. This was done by SDS-PAGE electrophoresis. These results for taq ligase are shown in figure 2 and the results for T5 exonuclease are shown in figure 3, below.

Figure 2. This gel shows elution fractions from the taq ligase purification. It can be seen that taq ligase is expressed in very large amounts in the fractions from induced cells(extremely dark band). It can also be seen that the elution is significantly more pure than the unpurified fractons. Although a smaller band of taq ligase can be seen in samples from the uninduced fractions, the expression in the uninduced cells can be attributed to leakyness in the T7 expression system