Abstract
Extracts of Escherichia coli convert [lambda] DNA to covalent circles. The reaction requires Mg ion and prior formation of H-bonded circles of the [lambda] DNA. Extracts of -sensitive, [lambda] -lysogenic and induced [lambda]-lysogenic strains and extracts of 2 recombination-deficient mutants were active. The covalent circles appear by sedimentation to be less twisted than those made intracellularly. Their sedimentation rate is sharply increased by addition of actinomycin.