Molecular mechanisms of genetic recombination in bacteriophage: II. Joining of parental DNA molecules of phage T4

Abstract

DNA was extracted from E. coli infected with a mixture of 32 P-labelled and bromouracil-labelled T4 phage particles after incubation for 60 min in the presence of 8 × 10 −3 M -KCN. This DNA was mixed with DNA extracted from cells infected with a mixture of 3 H-labelled and bromouracil-labelled T4 phage particles. The resultant mixture was centrifuged in a CsCl density gradient and fractionated. A considerable amount of 32 P without accompanying 3 H was found at a density intermediate between that of [ 32 P]DNA and bromouracil-labelled DNA. The complex with [ 32 P]DNA found in the intermediate fractions has the following properties: (1) the complex is composed of heavy and light components and is not a simple aggregate; (2) both components are sensitive to deoxyribo-nuclease and all the 32 P in the complex can be made acid-soluble by this enzyme; (3) the density of the complex does not change after treatment with ribonuclease; (4) the heavy and light components separate after heating at a temperature at which DNA extracted from bromouracil-labelled phage separates into single strands, but do not separate below the melting temperature of T4 phage DNA; (5) the analysis of the fragments produced by shear degradation indicates linearity of the complex as a whole. The complex is concluded to be a linear molecule composed of one 32 P-labelled “light” DNA and one bromouracil-labelled “heavy” DNA component joined end-to-end by hydrogen bonds. Structural models of the complex DNA and the possible genetic implication of formation of such molecules are discussed.