282 Raymond Latarjet 



also consider, in the case of T2, the possible inactivation of 

 a phage inhibitor (Sagik, 1954) which would influence the 

 apparent rate of inactivation of the phage itself. 



(2) A large phage like T2 can be inactivated by damage to 

 the DNA, or to the tail, or both. If DNA only is damaged, the 

 phage still attaches itself to its bacterial host, and kills it, 

 without multiplying. If some specific site at the tip of the 

 tail is knocked out the phage does not attach, or attaches in 

 such a fashion that it loses simultaneously its infective and 

 bactericidal powers. 



It is known that X-rays attack both sites at different rates 

 depending on whether the radiation acts through direct or 

 indirect effect. When it acts indirectly both sites have about 

 the same sensitivity, but when the direct effect becomes 

 more marked the relative resistance of the tail increases much 

 more than that of the DNA. For example, in T6 inactivated 

 by pure direct effect, the DNA is 28 times as sensitive as the 

 bactericidal activity (Latarjet and Fredericq, 1955). 



By treating a concentrated suspension of T2 in buffer 

 (10^^ particles per ml.) with succinic peroxide. Dr. Maxwell 

 found that both sites are attacked, and that the DNA is 

 about 2-5 times as sensitive as the other site. This ratio is 

 about the same as that found by Watson (1950) on T2 irradi- 

 ated by X-rays in • 8 per cent broth. It should be of interest 

 to examine this aspect of peroxide-treated phage. Arber and 

 Kellenberger (1955) have already observed some morphologi- 

 cal changes in T2 treated with hydrogen peroxide. 



(3) The sensitivities of different phages have been compared 

 in numerous experiments. The temperate phage X, which has 

 the same size as the virulent phage T2, but a smaller content 

 of DNA, is much more resistant to peroxide than the latter, 

 as it is more resistant to radiation. Among the virulent 

 phages of the T-series, T5 is the more resistant ; then come the 

 three T-even phages with about the same sensitivity. But, 

 contrary to what happens with radiation, the smaller phages 

 Tl, T3, T7 are more sensitive, possibly because of the lack of 

 a thick membrane around their DNA; for example, 10"* 



