III. BACTERIOPHAGE DNA AND BACTERIAL DNA 119 



down during lysis (Koch and Hershey, 1959j. Kellenberger concludes 

 that these "membraneless dense particles" represent DNA which has 

 been organized into a compact form as the result of the action of a 

 still unknown "organization principle" (Fig. 3). To what extent this 

 ability to form a folded configuration is a property of the DXA mole- 

 cule, and to what extent it relies on the presence of other molecules such 

 as the polyamines (Ames et al., 1958) or the internal proteins (Levine 

 et al., 1958) is a matter of conjecture, as is the role of the head mem- 

 brane, if present. It seems certain that the internal protein does not bind 

 to free DNA in the presence of 0.13/ NaCl, nor is it transferred to 

 progeny phage (^linagawa. 1961 1. It is of some importance to note 

 that proflavine, an acridine dye which is known to alter the structure of 

 free DNA (Lerman, 1961). also interferes with the intracellular organ- 

 ization of the compact form (Kellenberger et al., 1959). 



4. The Question of Single-Chain Regions 



The thorough studies of Sinsheimer (1959a,b) have led to the con- 

 clusion that the small bacteriophage. 0X174, contains a single molecule 

 of DNA which is not in the form of the Watson-Crick duplex. This 

 phage is about 10 times more sensitive to the decay of incorporated P^- 

 atoms, 10 times more sensitive to X-rays, and 25 times more sensitive 

 to UV than T2 phage (after correcting for the 77-fold difference in DNA 

 content) (Tessman et al., 1957; Tessman, 1959). This increased radio- 

 sensitivity is in accord with the hypothesis (Stent and Fuerst. 19601 

 that the lethal event in phage is the complete scission of the DNA 

 molecule; <^X, having only a single chain, would be expected to be 

 broken by every P^- disintegration. If increased radiosensitivity. and 

 in particular sensitivity to P^- decay, reflects single-chain construction 

 of the viral DNA, then these kinds of experiments could yield useful 

 information on the DNA in various phages. The only other phage known 

 to have a high sensitivity to incorporated P^- decay (also X-rays and 

 UV) is phage a studied by Graziosi and his collaborators (Celano et al., 

 1960). However, the DNA excised from this phage has all of the char- 

 acteristic properties of a duplex molecule such as that taken from T2 

 or T4 (Cordes et al, 1961; Aurisicchio et al., 1960). In this connection 

 it is important to note that the simple interpretation of P^- damage as 

 a "direct" effect needs revision in view of the fact that the efficiency of 

 killing can be decreased substantially by the addition of a free radical 

 trapping agent (Matheson and Thomas. 1960). 



A promising approach to the problem of detecting single-chained 

 DNA emerges from the studies of Mayor and Hill (1961). These authors 



