P22 BACTERIOPHAGE DNA MOLECULE .I. MATURE FORM

The DNA molecules isolated from mature particles of the temperate bacteriophage P22 are linear and double stranded, with a molecular weight of 26 to 27 million. The linear structure remains unaffected by annealing treatments of the sort that will cyclize the DNA molecules of the λ-related temperate bacteriophages. However, P22 DNA that has been partially degraded by exonuclease III does cyclize on annealing. This indicates a duplex terminal repetition of base sequence in P22 DNA, in contrast to the complementary single-stranded terminals found in λ. The length of the repeated region can be estimated to comprise 2.5 to 5.0% of the P22 DNA molecule. Full-length circular molecules can be produced also by denaturing and then re-annealing P22 DNA. This process is dependent upon complete separation of the two single chains on denaturation, and suggests that the DNA molecules released by individual P22 phage particles are circular permutations of one another. In support of this conclusion, competition-annealing experiments indicate that short fragments derived from the ends of P22 DNA contain all of the seqences present in a complete molecule. Therefore, the number of different cyclic permutations is large, and possibly equal to the number of base pairs in the genome. © 1968.