204 ROBERT L. SINSHEIMER 



section for T2, T4 or T6 is the same whether irradiated as extracellular phage par- 

 ticles, or irradiated shortly after injection. If densely ionizing soft X-rays are used, 

 however, the inactivation cross section shortly after injection is 1.6 times that of the 

 phage particles. This has been interpreted 90 as a result of uncoiling of the DNA to 

 a more extended target and consequent lesser waste of lethal hits in multiply hit 

 phage. 



A similar decrease of about 20% in sensitivity to incorporated P 32 -decay, im- 

 mediately after injection has also been attributed to an uncoiling of the DNA with 

 resultant lesser damage from the emitted jS-particle. 40 



Ultracentrifugal studies of the sedimentation properties of the parental 

 DNA (P 32 -labeled) in cells infected with T2 at low multiplicity have been 

 made by Watanabe et al. 91 The cells were disrupted by rapid decompression 

 at various times after infection. Using a partition centrifuge cell, measure- 

 ments were made of the radioactivity remaining above the partition after 

 various periods of centrifugation. From the data, estimates could be made 

 of the distribution of the radioactive components among five ranges of 

 sedimentation rate. 



The results indicated that about 10% of the radioactivity continued to 

 sediment at a rate characteristic of intact virus particles, presumably repre- 

 senting phage which adsorbed but failed to inject. The bulk of the activity 

 (60%) sedimented at a rate characteristic of free DNA (S = 10-20). Ap- 

 proximately 25 to 35% of the activity appeared to have a very low sedi- 

 mentation rate, less than 0.5 S. A similar proportion of the activity was 

 found to be acid-soluble. 



The bulk of the injected parental DNA, then, appears to be present as 

 essentially free DNA in the infected cell. No indication was obtained of 

 any enduring attachments of the parental DNA to bacterial structures, 

 although such structures would have had to be rather large to be evident 

 by this method. 



The proportion of parental DNA that has apparently been degraded to 

 the extent of acid solubility seems rather larger in this experiment than 

 in experiments of other workers. Hershey and Burgi 61 found less than 10%. 

 To some extent this undoubtedly represents a degradation of the DNA of 

 superinfecting phages. 92 Within about 2 minutes after infection with T2, the 

 infected cell is "immune" to superinfection by other phage, even if the 

 latter differ by only a single genetic trait. Lesley et a/. 92 demonstrated that 

 this immunity was in part the result of a mechanism established shortly 

 after infection that results in the extensive degradation of the DNA of any 

 superinfecting particles. 



91 I. Watanabe, G. S. Stent, and H. K. Schachman, Biochim. et Biophys. Acta 15, 

 38 (1954). 



92 S. M. Lesley, R. C. French, A. F. Graham, and C. E. van Rooyen, Can. J. Med. 

 Sci. 29, 128 (1951). 



