376 S. GAUD AND O. MAAL0E 



reactions, such as rupture of peptide, disulfide, or pyrimidine bonds is of the 

 order of 10"^^ ^nd it was suggested that, perhaps, the TMV particles contained 

 about 50 critical bonds, the rupture of one of which by absorption of a 

 quantum of UV causes inactivation. Since the action spectrum of TMV does 

 not particularly point to the nucleic acid as the principal photosensitive 

 element, it is difficult to guess what type of bond might be involved. Oster 

 and McLaren summarized their discussion as follows: ". . . these approxi- 

 mate calculations simply serve to show that the quantum efficiency for 

 destruction of nucleic acids as well as for protein linkages is much higher than 

 for inactivation of viruses. The nucleic acid protein framework of the virus 

 apparently serves to hold together the radicals formed by quanta and thus 

 allows for a high probability of recombination without net chemical change 

 and with an over-all low quantmn efficiency." 



In this connection it is perhaps significant that infective RNA prepared 

 from TMV is about six times as sensitive to UV as the RNA protein complex 

 of the intact virus particle (McLaren and Takahashi, 1957). 



The coliphages Tl and T2 were thoroughly studied by Zelle and Hollaender 

 (1954). For both phages, the action spectrum was determined and the quan- 

 tum yield calculated for 10 different wavelengths in the interval between 

 2200 and 3000 A. Over this whole range reasonably constant values of about 6 

 and 3 X 10~* were obtained for Tl and T2, respectively. Fluke (1956) carried 

 out similar experiments with dried Tl. His estimate of the quantum yield 

 was 3 X 10~*, but his action spectrum (Fluke and Pollard, 1949) differs from 

 that of Zelle and Hollaender for the shortest wavelengths and, in that region, 

 the quantum yield therefore does not remain constant. Zelle and Hollaender 

 (1954) suggest that nonspecific absorption by protein contained in the dry 

 film perhaps accounts for this discrepancy. 



The more or less constant quantum yield registered in the 2000 to 3000 

 A region indicates that inactivation of Tl and T2 phages is due to UV 

 absorption in a particular substance; and, as mentioned, the action spectrum 

 already strongly suggested that this substance is the virus nucleic acid. The 

 theory of the action spectrum and the assumptions involved in interpreting 

 data of this kind have been detailed in a review by Loofbourow (1948). 



Bawden and Kleczkowski (1955) have remarked that the rule of McLaren 

 (1949), that the quantum yields for proteins are approximately inversely 

 proportional to the molecular weight, does not apply to TMV, which is too 

 sensitive according to this rule. They ascribe the relatively high sensitivity to 

 the presence of nucleic acid in the virus. For phage T2 the deviation from 

 McLaren's rule is much more striking: the quantum yield is 10 times that of 

 TMV and, at the same time, T2 is bigger than TMV. In T2, it is 

 reasonably certain that it is the nucleic acid that determines the sensitivity 

 toUV. 



