Parental to Progeny Molecular Recombination

Parental deoxyribonucleic acid (DNA) molecules of bacteriophage T4 recombine with their progeny in a way which disperses pieces of the parental molecule over large numbers of the progeny DNA molecules. The parental contribution per progeny phage, or DNA molecule, which receives parental label, is about 5 to 7% of the phage DNA molecule (A. W. Kozinski, Virology 13:124, 1961). The fragment is inserted as a single piece with covalent repair at the intersection of the parental piece with the progeny polynucleotide chain (A. W. Kozinski and P. B. Kozinski, Virology 20:213, 1963; E. Shahn and A. W. Kozinski, Virology 30:455, 1966). Breakage and rejoining ofDNA as a mode of recombination of bacteriophage has been proven so far only for T4 and X (A. W. Kozinski, Virology 13:124, 1961; M. Meselson and W. 0. Weigle, Proc. Natl. Acad. Sci. U.S. 47:857, 1961). The purpose of this communication is to report that T7 behaves in a manner similar to T4; i.e., parental DNA becomes fragmented and inserted as a subunit covalently bonded to the adjacent progeny strand. Analysis of the mode of parent-to-progeny transfer and recombination for bacteriophage T7 was conducted by use of analytical procedures and media similar to those described for T4 (A. W. Kozinski et al., J. Virol. 1:758, 1967). Escherichia coli B23 was grown to 3 X 108 bacteria/ml in heavy (5-bromodeoxyuridinesubstituted) medium and was infected with 32p_ labeled (specific activity, 5.0 mc/mg) T7 bacteriophage at a multiplicity of infection of 3.0. Six minutes after infection the bacteria were sedimented to eliminate unadsorbed phage and resuspended in fresh heavy medium. (This was necessary as it was found that 20 to 50% of the parental phage were not adsorbed. The unadsorbed phage did not differ in density from adsorbed phage, though in a separate test they failed to adsorb to freshly added bacteria.) The bacteria were incubated until lysis (approximately 35 min), and the progeny phage were purified. The amount of parent-to-progeny transfer was estimated as follows: after washing the bacteria free of unadsorbed phage, a sample of the infected bacteria was precipitated with trichloroacetic acid and considered as 100% recovery of the parental label; samples were taken for acid precipitation at each successive step of purification. Carrier phage was added after treatment of lysates with deoxyribonuclease and was plated at each step of purification thereafter; the ratio of 32p to plaque-forming units (PFU) was calculated at each step of the procedure. Although there were substantial losses of material during purification (Table 1), the specific activity of 32P/PFU remained essentially constant;