RADIATION AND VIRUSES 347 



recognizable events in the interaction of a virus with the host cell, the 

 simultaneous loss of two properties may be taken as an indication that the 

 corresponding events result from the same step in interaction. Thus, 

 successive steps in host-virus interaction can be traced by the analysis of 

 the residual properties of virus particles exposed to different radiations 

 under different conditions. As an example, this type of analysis as 

 carried out for bacteriophages T2, T4, and T6 active on the common host 

 Escherichia coli B will be described. 



The major phases of interaction between these viruses and their 

 common host are fairly well known (for reviews, see Delbriick, 1942; 

 Cohen, 1949; Luria, 1950; Benzer et al., 1950). One or more active par- 

 ticles of bacteriophage become adsorbed by the susceptible bacterium; the 

 adsorption is irreversible under the usual environmental conditions. 

 Reproduction of the bacterial cell is stopped, and there is complete sup- 

 pression of the synthesis of the specific components of bacteria, in par- 

 ticular, of bacterial enzymes. There occurs a quick and profound change 

 in the cytologically recognizable nuclear apparatus of the bacterial cell, 

 which in fixed preparations appears to be disrupted and is later replaced 

 by a fine, granular material giving the cytochemical reactions of deoxy- 

 ribonucleate and probably representing the new virus. All synthetic 

 processes in the infected cell are directed toward the synthesis of bacterio- 

 phage components — phage protein and phage nucleic acid — through the 

 activity of preexisting bacterial enzymes. After a rather precisely 

 defined latent period, during which the synthesis of phage components is 

 followed by the appearance of large numbers of new phage particles, the 

 bacterial cell is lysed and releases the new phage into the medium. 



The outcome of the infection also depends on the number and genetic 

 constitution of the infecting particles. If too many particles are present, 

 there may occur a "lysis from without," apparently resulting from a 

 massive damage to the bacterial surface. This type of lysis takes place 

 without phage reproduction and without disruption of the bacterial 

 nuclei. The T-even bacteriophages, in their wild types, also exhibit a 

 phenomenon of "lysis inhibition," i.e., a delay in lysis if two or more par- 

 ticles infect a bacterium at an interval of several minutes. 



Let us see what happens if active bacteriophage particles are replaced 

 by particles inactivated by exposure to radiation. 



Phage particles inactivated by moderate doses of ultraviolet radiation 

 (N/No > e~^^, by extrapolation), if tested under conditions where no 

 reactivation occurs (see Sect. 4-2), are still capable of being adsorbed by 

 bacteria and of killing the bacterial cell (Luria and Delbriick, 1942). 

 Bacterial nuclei are disrupted and bacterial syntheses are suppressed, but 

 no synthesis of phage components takes place. Lysis and liberation of 

 active bacteriophage are absent; even if the infected cells arc artificially 

 l)roken, no active bacteriophage is extracted (Luria and Human, 1950). 



