Restoration by T4 ligase of DNA sequences sensitive to "flush" cleaving restriction enzyme.

Fouteen "flush"-ended segments originate from the action of the restriction endonuclease Hae III of Haemophilus aegiptius on the DNA of the colicinogenic factor ColE 1 (A. Oka and M. Takanami, Nature, 264, 191, 1976). They are joined by the T4 polynucleotide ligase. The reaction can be monitored by gel electrophoresis, electron microscopy and resistance to phosphatase of the 5'-32P labelled ends. The joined products are a random recombination of the original segments, and can be cleaved by the same Hae III endonuclease to restore the exact electrophoretic pattern of the Hae III-cut ColE 1 DNA. In a properly diluted mixture of 5'-32P segments treated with T4 ligase, the level of phosphatase resistance is very close to the frequency of circle-formation as determined by electron microscopy: thus, the joining of the "flush"-ends involves the formation of circular structures covalently closed in both strands.