Fractionation of deoxyribonucleic acid from phage-infected bacteria

1. DNA has been isolated in 90% yield from T5-infected cultures of Escherichia coli ;pulse'-labelled with [(3)H]thymidine. It had a buoyant density in caesium chloride solution identical with the DNA of mature T5 phage, and no components of unusual buoyant density were detected. 2. The DNA preparation was resolved into two major components of differing specific activity on a column of kieselguhr coated with methylated serum albumin. The DNA of high specific activity could be eluted from the column only with 2n-ammonia, and the firm binding did not appear to be due to an artifact of preparation. 3. A similar fractionation into two DNA components of differing specific activity was observed when the ;pulse'-labelled culture was lysed with sodium dodecyl sulphate and the lysate rocked with phenol. The DNA of high specific activity was found in the interface precipitate between the phenol and aqueous layers. 4. The amounts of DNA in the two fractions were measured at different times after infection and the radioactivity content of each was determined at various times after a short ;pulse' of [(3)H]thymidine. The interface fraction contained the replicating phage DNA, and the DNA from mature phage particles appeared in the aqueous fraction. 5. Analogous results were obtained with T2-infected E. coli. In the presence of chloramphenicol the DNA in the interface fraction was not converted into DNA extractable into the aqueous layer. Since chloramphenicol prevents the condensation of DNA into phage heads, it is suggested that any DNA in extended configuration is trapped inside the rigid-layer framework of the cell wall. 6. Treatment with lysozyme released much of the DNA from the interface precipitate. This DNA was firmly bound by the chromatographic column and had the same buoyant density in caesium chloride solution as normal T5-phage DNA. Sucrose-gradient sedimentation studies showed that it was heterogeneous and that as much as 60% sedimented faster than T5-phage DNA.