272 G. S. STENT 



than the T-even group illustrates this difficulty, since it is by no means clear 

 whether this observation really reflects the appearance of phage DNA or only 

 the continuation of host DNA anabolism (Crawford, 1957).) 



A few radioactive tracer experiments have been carried out with other 

 phage-host systems. The bacterial contribution to the phage nucleic acid, for 

 instance, has also been studied for strains Tl, and for the two related strains, 

 T3 and T7, by removal or addition of a labeled isotope at the moment of 

 infection of the bacterial culture (Labaw, 1951; Putnam e^ al., 1952). Here, the 

 bacterial contribution is greater than that to the T-even strains, since phage Tl 

 derives about 56 % of its phosphorus, and T3 and T7 from 60-90 % of tlieu- 

 phosphorus, from materials already present in the host cell at the moment of 

 infection. Kinetic assimilation studies of the atoms of the bacterial contribu- 

 tion similar to those already described for the T-even strains show that here 

 too, the host nucleic acids are provenance of the bacterial contribution 

 (Labaw, 1953), although the relative importance of the contribution from 

 bacterial RNA and bacterial DNA has not yet been assessed. The bacterial 

 contribution to the DNA of phage T5 has been reported as 30 % i.e., similar 

 to that of the T-even strains (Labaw, 1951). Transfer measurements have also 

 been carried out with P^-'labeled stocks of T3 and T7, which showed that from 

 30 to 50 % of the parental labeled atoms reappear among the progeny in 

 each case (Kozloff, 1953; Watson and Maaloe, 1953). Hence, also, the transfer 

 of parental nucleic acid to progeny can be considered to be a general phenome- 

 non associated with phage reproduction. It must be noted with regret, 

 however, that at the time of writing, the blendor experiment of Hershey and 

 Chase (1952), that great milestone which signaled the germinal role of the 

 phage DNA, has not yet been repeated successfully with any phage outside 

 the T-even group. 



There is one aspect, however, in which the T-even phages rather differ from 

 many other bacteriophages; that is the way in which they affect the host cell 

 in the course of their reproduction. Infection with T-even phages immediately 

 arrests bacterial growth, as measured by either the turbidity or rate of 

 respiration of the infected culture (Cohen and Anderson, 1946), and suppresses 

 formation of induced bacterial enzymes (Benzer, 1953). (Such interference 

 with induced enzyme synthesis was actually first discovered by Monod and 

 Wollman (1947) with another coliphage, and later also found to obtain with 

 the pyocyanea phage P2 (Jacob, 1951).) Intracellular multiplication of phage 

 strains, like the pyocyanea phage P8 (Jacob, 1952) and the megatherium phage 

 899 (Siminovitch and Rapkine, 1952; Siminovitch and Jacob, 1952), does not, 

 on the other hand, produce such drastic effects and allows the infected 

 bacteria to increase in size and synthesize respiratory as well as induced 

 enzymes. Cytological examination of phage-infected bacteria reveals that 

 while growth of the T-even phages quickly disrupts the host cell nucleus, the 



