in. BACTERIOPHAGE DNA AND BACTERIAL DNA 121 



Fraenkel-Conrat and Williams, 1955). This is as might be expected in 

 view of the fact that the polyribonucleotide chain is completely sur- 

 rounded by the protein subunits in the completed TMV particle (Klug 

 and Caspar, 1960). 



This experience with TMV leads one to inquire whether the organ- 

 ization of the phage head is strongly dependent on the enclosed nucleic 

 acid. The globular form and the large amount of DNA to be enclosed 

 suggests that not all of the DNA can be in contact with the protein sub- 

 units of the head membrane. The existence of "density mutants" in 

 phage lambda indicate that this phage can exist in a number of different 

 foiTns differing in DNA content (Kellenberger et al., 1961a,b). A vari- 

 able DNA content seems to be displayed by the different strains of 

 A-dg transducing phage (Weigle, Meselson, and Paigen, 1959). Likewise 

 the existence of deletion mutations in T4, which seem to be missing a 

 segment of the linkage map, suggests a variability in DNA content 

 (Nomura and Benzer, 1961). One would expect that this difference in 

 DNA content would result in a significant perturbation in the packing 

 arrangement of the DNA. The fact that viable phage particles can be 

 constructed in spite of this alteration in DNA structure suggests that 

 there is some degree of structural independence between the compact 

 nucleic acid and the protein membrane. 



In conclusion, it seems clear that the DNA inside the head of bac- 

 teriophage has a special organization, but the requirements for this 

 organization are not overly strict because phage particles can be con- 

 structed with slightly variable amounts of DNA. The studies of Kellen- 

 berger would indicate that the folding and "condensing" of the DNA 

 precedes the assembly of the head membrane around the packet of 

 nucleic acid. As to the details of the folding and how folding allows for 

 rapid injection, we remain ignorant. 



B. THE STRUCTURE OF THE LIBERATED PHAGE DNA MOLECULE 



1. The DNA jrom <f>Xl74 



The first successful isolation and characterization of the DNA mole- 

 cule liberated from any bacteriophage was accomplished by Sinsheimer 

 (T959a,b). Measurements of the DNx\ content by colorometric tests 

 indicated the virus contained 25.5% DNA. Light-scattering molecular 

 weight determinations yielded a value of 6.2 X lO*' daltons for the intact 

 virus. This requires the DNA content to be 1.6 X lO'' daltons. The DNA 

 was extracted and purified by phenol extraction and the molecular 

 weight repeatedly measured by light scattering under various conditions, 

 giving values between 1.6 and 1.8 X 10'\ By assuming the absence of 



