The nature of the transformation process in Escherichia coli K12

Summary The nature of the transformation process in E. coli was studied by investigating various factors which affect the efficiency of transformation. CaCl₂ treatment of the recipient cells is absolutely necessary for transformation and the optimum concentration was found to be 30 mM. The efficiency of transformation is dependent upon temperature during incubation of the recipient cells with DNA. The efficiency is also affected by the molecular weight of donor DNA used. Sheared DNA with molecular weights ranging from 10 to 30x10⁶ daltons was most efficient, increasing the number of transformants by a factor of 5 to 10 as compared to unsheared DNA. The intracellular status of recB-recC DNase (ATP-dependent DNase) is another important factor which determines the transformability of E. coli K12. This was shown by demonstrating that a recB ^- recC ^- sbcA ^- strain was transformable as well as the previously demonstrated recB ^- recC ^- sbcB ^- strain. Therefore, it seems reasonable to conclude that the E. coli K12 strain is transformable if the ATP-dependent DNase is absent or diminished in function and a state of recombinational proficiency exists.