Cotransformation of linear chromosomal DNA and plasmid DNA in Escherichia coli

Transformation of Ca²⁺-heat shocked Escherichia coli cells with plasmid DNA has become an indispensable technique in the study of plasmids and in genetic engineering. The process of induction of competence by Ca²⁺-heat shock is still poorly understood, but it has been argued that only a small proportion of the shocked cells is effectively rendered competent in the process. This has been deduced from the fact that in transformation with two different plasmids as donor DNA, the observed frequency of double transformants is higher than would be expected from the observed transformation frequencies of each of the plasmids separately