Prolonged incubation in calcium chloride improves the competence of Escherichia coli cells.

Escherichia coli cells are 4--6 times more transformable and 20--30 times more competent after 24 h incubation in cold calcium chloride than immediately after calcium chloride treatment. With 24-h-old competent cells we obtained routinely 2 . 10(7) transformants per microgram of pBR322 DNA, and transformed over 20% of viable cells.