THE SYNTHESIS OF PROTEINS 23 



course, be assumed (cf. Gratia, 1938) that infection occurred sometime 

 in the past, but there is no direct evidence for this assumption. No means 

 is known to free lysogenic strains from their viruses. 



It would be expected, then, that if an organism could be completely- 

 freed from a protein, it would no longer produce that protein until 

 reinoculated with it. The suggestion was made (Northrop, 1938) that 

 the substance producing type-specificity in pneumococci was an example 

 of such a reaction. The analogy between virus and the substance regu- 

 lating type-specificity in pneumococci was recognized by Gratia (1936). 

 Since then the substance has actually been isolated (Avery, MacLeod, 

 and McCarty, 1944). It turns out that it is a desoxyribonucleic acid 

 instead of a protein, but the mechanism of formation may well be the 

 same. 



Virus proteins possess the following characteristics which must be 

 accounted for by any hypothesis for their formation : 



1. They increase only in the presence of appropriate living cells. 



2. Production of virus is usually associated with growth and 

 active metabolism of the host cells (Bordet and Jaumain, 1921 ; 

 Krueger and Northrop, 1930; Zinsser, 1937; Gratia, 1938; 

 Howe and Mellors, 1945, this paper contains other relevant 

 references). 



This correlation between metabolism and virus production is strong 

 evidence that the synthetic part of the reaction is carried out by the cell 

 in conjunction with the synthesis of normal proteins, since the viruses 

 themselves do not contain the enzymes necessary to carry out these re- 

 actions. 



If it should be found that virus production can take place only during 

 cell division, the analogy between virus and gene suggested by many 

 workers (cf. Beadle, 1945) would be greatly strengthened, since genes 

 also multiply only during cell division, whereas structural proteins are 

 formed continuously. 



All hypotheses concerning the production of virus proteins (other 

 than those which consider the virus to be a cell) assume some sort of 

 autocatalytic reaction. Some writers (Troland, 191 7; Stanley, 1938b; 

 Caspersson, 1939; Jordan, 1944) assume that the virus can synthesize 

 itself from simple building stones. This mechanism requires energy and 

 entails a series of assumptions as to the source of energy and the manner 

 in which it is made available (cf. page 10). 



Bordet (1931) pointed out that a simpler and equally adequate hy- 



