ANIMAL VIRUSES! A COMPARATIVE SURVEY 13 



of potentiality for special association with lipids or mucoproteins as some 

 cell proteins possess. Along such lines one could think of the sequence of 

 increasing incorporation of host cell components seen in the series polio- 

 virus, arborvirus, and myxovirus. 



Such a picture will undoubtedly be discarded as knowledge of the synthetic 

 mechanism of cells increases. But we can be certain that, with each develop- 

 ment in the field of macromolecular chemistry, it will become clearer where 

 the intrusion of the virus into the cell affects the metabolic processes. It is 

 equally evident that the behavior of viruses may provide one of the most 

 important tools for the continuing exploration of the broader field of protein 

 and nucleic acid in general. These two statements are virtual truisms, but it 

 is less widely recognized that the behavior of viruses may be equally relevant 

 to the understanding of the differentiation of parts and morphological control 

 within the cell. There are many hints, in addition to those mentioned above, 

 that there are special relationships between the host cell surface and virus 

 infection. One way of looking at both the proliferative stimulation provided 

 by some types of virus infection — the Shope fibroma, for example — and the 

 oncolytic effect of other viruses is to postulate specific effects at the cell 

 surface (Burnet, 1957). In this general field, the differences between the 

 various groups of animal viruses may be more illuminating than their 

 resemblances. 



Although only this sketchy outline is possible for the RNA-containing 

 viruses, the position with the DNA viruses is even worse. It may well develop 

 that analogies to bacteriophage behavior will be helpful in the interpretation 

 of viruses like the adenoviruses and Shope papilloma virus. There is, however, 

 the very important gap in phage studies that we do not know with certainty 

 whether or not phage protem pattern is determined directly by DNA or 

 through some type of intermediary mechanism involving protem, RNA, or 

 both. From the point of view of fruitful analogy with animal viruses, there is 

 the further disadvantage that T-even phages, which have alone been really 

 intensively studied, possess DNA uniquely different in its content of 5- 

 hydroxymethvl-cytosine and glucose from any other DNA in the whole range 

 of life. 



The approach to an understanding of the relationship of the larger viruses 

 of the psittacosis and poxvirus groups to the infected cell will probably draw 

 more from bacterial physiology than from the behavior of the smaller RNA 

 viruses. Both types have a clearly demonstrable limiting membrane and a 

 central body of nuclear character containing DNA. In the closely studied 

 poxviruses, Peters (1957) comes to the conclusion that, despite the disparity 

 in size, the general structure of the vaccinia virus nucleus and its reactions to 

 enzymatic treatment is closely reminiscent of bacterial nuclei, such as those 

 in spores of Bacillus megatherium. If this is the case, these viruses will have 



