Artificial Transformation of Bacterial genome

Artificial Transformation:

For a long time, E. coli — a very important organism employed as a model in genetical and molecular biological research — was thought to be not amenable to transformation, because this organism is not naturally transformable.

It has been discovered later that E. coli cells can also be made competent to take up exogenous DNA by subjecting them to special chemical and physical treatments, such as high concentration of CaCl2 (salt-shock), or exposure to high-voltage electric field. Under such artificial conditions, the cells are forced to take up foreign DNA bypassing the transport system operating in naturally transformable bacteria. The type of transformation occurring in E. coli is called artificial. In this process, the recipient cells are able to take up double-stranded DNA fragments which may be linear or circular.

In case of artificial transformation, physical or chemical stress forces the recipient cells to take up exogenous DNA. The incoming DNA is then integrated into the chromosome by homologous recombination mediated by RecA protein.

The two DNA molecules having homologous sequences exchange parts by crossing over. The RecA protein catalyses the annealing of two DNA segments and exchange of homologous segments. This involves nicking of the DNA strands and resealing of exchanged parts (breakage and reunion).

A generally accepted model explaining homologous recombination is diagrammatically shown in Fig. 9.98: