Fractionation of deoxyribonucleic acid from phage-infected bacteria

Multiple origins and circular structures in replicating T5 bacteriophage DNA

Replicating T5 phage DNA was gently isolated using NaI density gradient centrifugation and examined by electron microscopy. At the beginning of phage DNA synthesis, linear unit-length T5 DNA molecules containing from one to four replicating "eye-loops" were consistently observed. Replication in these molecules was found to proceed bidirectionally from multiple, internal origins. A primary origin of replication is located near the center of the T5 genome, which does not coincide with the location of any of the nicks (single-strand breaks) found in mature T5 DNA. The initiation of replication at the various origins within an individual molecule does not appear to follow any definite temporal sequence. At later times in the infection, we have observed a significant number of circular T5 DNA molecules-both replicating and nonreplicating-whose average circumference is approximately the length of mature T5 DNA minus the terminal redundancy. The replicating circular molecules appear to be either in a theta configuration, a sigma configuration with the tails all being less than the length of the circle, or a combination of theta and sigma forms.

The process of infection with bacteriophage phiX174. XXX. Replication of double-stranded phiX DNA

Intermediates involved in the replication of double-stranded varphiX174 RF DNA have been identified and partially characterized. Analysis of pulselabeled RF DNA suggests that the synthesis of progeny RF molecules involves, in part, the addition of nucleotides to linear complementary strands on a circular parental strand as template, so as to produce intermediate DNA strands of greater than viral length. Electron microscopy reveals DNA rings with "tails" and "double rings," which could be the intermediate structures. A model is postulated for the replication process.

Deoxyribonucleic acid replication in lambda bacteriophage mutants

After ultraviolet light induction of Escherichia coli K-12 strain W3350(lambda), several structural intermediate forms of phage deoxyribonucleic acid (DNA) are synthesized. The early defective lysogens of lambda, sus O(8), sus P(3), and T(11), were found to synthesize none of the DNA structural intermediates. A lysogen believed to be defective in all known phage activities, lambdasus N(7), was found to be able to synthesize an early phage DNA intermediate. The lysogen lambdasus Q(21), defective in late phage functions, is able to synthesize the early phage DNA intermediate and a concatenated molecule of greater molecular weight than the mature lambda DNA.

Some properties of DNA from phage-infected bacteria

Replicating T5 or lambda phage DNA has been labeled by adding tritiated thymidine for short periods to cultures of phage-infected Escherichia coli before isolation of intracellular DNA. Two procedures are described for separating T5 replicating DNA from DNA of intracellular phage particles. Both T5 and lambda replicating DNA had the same bouyant density in cesium chloride as DNA from phage particles but sedimented faster when centrifuged in sucrose density gradients. The fast sedimentation did not appear to be caused by DNA protein or DNA-RNA complexes or by aggregation of DNA, but is probably due to DNA molecules of unusual structure. Experiments involving hydrodynamic shear and sucrose density gradient centrifugation at alkaline pH have suggested that with lambda the replicating form of DNA is a linear molecule considerably longer than the DNA molecules of lambda-phage particles. The constituent polynucleotide chains of lambda but not T5 replicating DNA also appear to be longer than those of phage DNA.