A detailed restriction endonuclease map of Escherichia coli phage N4 DNA is presented. The cleavage sites for restriction endonucleases HaeII (1 cut), HhaI (3 cuts), XbaI (5 cuts), AvaII (15 cuts), and HpaI (25 cuts) have been determined by (1) digestion with endonucleases after end labeling of the DNA; (2) digestion with 2 endonucleases; (3) digestion of N4 DNA after λ exonuclease treatment; and (4) annealing of isolated fragments to nitrocellulose blots containing fragments obtained by digestion with a different restriction endonuclease. N4 DNA was determined to contain 71 kilobase pairs. Cross-hybridization of the terminal fragments suggested the presence of terminal repetition. This observation was confirmed by electron microscopy of N4 DNA molecules annealed after λ exonuclease digestion. N4 DNA contains direct terminal repeats approximately 500 base pairs in length.
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