Studies on the replication and transfer to progeny of the DNA of bacteriophage T4

Abstract

The purpose of the present research has been to learn more about the mechanism of replication and ultimate fate of the DNA of the phage T4. The distribution of parental material in DNA molecules in the replicating pool and in the DNA obtained from progeny phage has been examined. The association of parental atoms with the r genotype has also been examined. Phage in which the nitrogen and carbon we re replaced by the stable heavy isotopes N15 and C13 were used to infect unlabeled bacteria. At various times after infection, aliquots of the culture were removed and lysed; finally, progeny phage were harvested and lysed. DNA molecules containing heavy atoms show a corresponding increase in their density. Separation is achieved by density-gradient centrifugation. Most parental atoms transferred to progeny are found in DNA of light density. Following fragmentation, DNA containing parental atoms is found to have shifted to densities between those of light and half-heavy DNA. In the replicating pool, half-heavy DNA is also found. It is concluded that transferred DNA is incorporated into hybrid units which are attached to larger segments of completely new DNA. The parental atoms may comprise a part of one strand of a duplex DNA molecule. An analysis was made of the density distribution of the viable progeny of a multiple infection with T4r and r+, one of which was labeled with N15 and C13. At densities greater than that of the average progeny, relatively more progeny of the genotype of the labeled parent are found. Thus, parental atoms tend to remain associated with parental genotype.