70 S. BENZER [vol. 63 



enables us to discern the existence of multiple intracellular entities at a time when 

 fully infective phages are not yet detectable with the Doermann technique. 



The anomalous results with T2r may be of far greater interest, however. 

 During the critical phase between the entrance of one infecting particle and the 

 appearance of intracellular progeny, enormous changes are reflected by the 

 survival curves. It seems likely that these resistance changes may bear a close 

 relation to the phenomenon of multiplicity reactivation. While the resistance 

 changes do not tell us what is actually happening, they at least give us something 

 easy to measure which serves as an empirical index to development. The value 

 of this index has already been demonstrated previously in the justification of 

 the techniques used in these experiments (adsorption in buffer, chilling to stop 

 growth). 



In the case of T2r, the maximum change in slope is by a factor of over 20 (free phage 

 compared with t = 10 minutes). As pointed out by Luria and Latarjet, intracellular accumu- 

 lation of ultraviolet-absorbing materials (nucleic acid components) must be considered as 

 a possible contributing factor. 



By microspectrography of uninfected cells (at 2570 A) Hed^n (1951) finds an average 

 extinction through the thickness of a cell of around 0.1, corresponding to 80 per cent trans- 

 mission. A phage particle which is adsorbed on the surface of a bacterium is therefore 

 shielded from ultraviolet to the extent of 20 per cent in one direction. This should lead to 

 an average increase by only 10 per cent in the resistance (at i = 0) compared with free 

 phage (since the cell rotates in all directions during exposure). Penetration of the phage 

 into the cell should have little effect on the magnitude of this average shielding. In order 

 to account for a subsequent change by a factor of 20 in the intensity of ultraviolet reaching 

 the phage, a coating of nucleic acid, 1 ^ in diameter, would have to be produced. The meas- 

 urements of Luria and Human (1950) and Cohen and Arbogast (1950) on optical density and 

 nucleic acid content in suspensions of cells multiple infected with T2 reveal increases during 

 growth, but these are far too small to account for the observed resistance changes during 

 the first 10 minutes. 



In the case of T7, however, the small change in slope may well be due to a screening 

 effect. 



A plausible interpretation of the increase in resistance of T2r complexes during 

 the first half of the latent period may be the following: A T2r particle, after 

 adsorption to a sensitive bacterium, must undergo a series of successive steps 

 A^B— >C— ^D-^, etc. in the course of reproduction. Each of these steps has 

 a certain cross section for being blocked by ultraviolet (e.g., by inactivation of 

 an enzyme which is concerned with the step). Blockade of any one of these steps 

 prevents normal development and causes inactivation of the phage. At time 

 zero, the total cross section of the phage is therefore the sum of these individual 

 cross sections, and the survival curve is exponential. As development proceeds, 

 the steps which have already been passed are no longer needed, and the effective 

 cross section decreases progressively while inactivation of the remaining steps 

 retains the characteristics of a one-hit phenomenon. 



The absence of this behavior in the case of T7 suggests that there may be 

 great differences in the mode of reproduction of T7 and T2r. These two phages 

 are also dissimilar in other respects. T2r is a relatively large particle with a 

 head and a tail and appears to have a kind of membrane (Anderson, 1949); it 



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