514 G. H. BERGOLD 



treatment of the virus particles and by ultracentrifugation, have less N 

 (12.5 %) than the virus particles (13.9 %). 



TABLE III 



Nitrogen, Phosphorus, and Deoxyribonucleic Acid (DNA) Content of Virus 

 Membranes, Virus, and Polyhedral Protein" 



Polyhedral Virus 



protien (%) Virus (%) membrane (%) 



N 15.5 13.9 12.5 



P*' 0.064 0.915 0.45 



DNA" — 7.9 O.S'^ 



" After Bergold and Wellington, 1954. 



^ Determined by the strychnine phosphomolybdate method (Bergold and Pister, 

 1948b). 

 " Determined by the indole method (Ceriotti, 1952). 

 '^ Probably due to contamination by virus to the extent of about 10 %. 



The first report of the presence of purine bases m B. mori polyhedra came 

 from Mammta (1940); intensive investigations (Smith and Wyatt, 1951; 

 Wyatt 1952a,b) of the bases of several msect viruses are smnmarized in 

 Tables IV and V. The rod-shaped nuclear insect viruses contain only the 

 purines (adenine and guanine) and the pyrimidines (cytosine and thymine), 

 but no 5-methyl-cytosine or uracil. It is interesting that the ratios of adenine 

 to thymine and guanine to cytosine are almost constant, but the ratio of 

 adenine + thymine to guanine + cytosine is variable. The differences are 

 apparently not at random but occur in steps (0.7; 0.95-1.10; 1.34-1.36; 1.67; 

 and 1.87), representing groups of similar or identical ratios (see Table V). 

 The significance of these steps is not known. As expected, polyhedron and 

 capsule virus of closely related hosts {Malacosoma disstria Hbn. and Mala- 

 cosoma americanum (F.), and Choristoneura fumiferana (Clem.) and C. muri- 

 nana (Hbn.)) have similar ratios. It is surprising, however, that polyhedron 

 viruses of widely separated lepidopterous hosts, a polyhedron virus from a 

 hymenopteron (N. sertifer), and a capsule virus of a lepidopteron (C 7nuri- 

 nana) have similar AT/GC ratios. This may suggest that not all DNA is of 

 genetic specificity. It is noteworthy that representatives of two different 

 groups of polyhedron and capsule viruses have entirely different ratios, 

 although they occur in the same host (C. fumiferana). 



Little is known about the chemical composition of cytoplasmic polyhedra. 

 A preliminary test of such polyhedra from C. fumiferana was positive for 

 DNA (Bird and Whalen, 1954). However, an analysis of purified spherical 



