368 F. W. STAHL 



regard to our notion about the nature of multiplicity reactivation, these 

 results are encouraging in two respects: (1) They indicate that a phage 

 particle inactivated by ultraviolet light can perform a function on behalf of 

 other particles within the cell. (2) The function of a cistron can be inactivated 

 by ultraviolet light at a rate approximately that expected for a vulnerable 

 center, i.e., 0.4/3 of the rate of phage inactivation. 



An extension of the above experiments show that hits which inactivate 

 the function of the rll cistrons are (at least in part) identical with those 

 which knock out a marker in cross-reactivation experiments. Among those 

 cells which liberated phage in the function-inactivation experiments, Krieg 

 (1958) scored for the cross reactivation of the r+ marker. He found that the 

 probability of cross reactivation among these cells became constant at a 

 value of 0.14 (for markers in the B cistron). The knockout of a remote marker 

 in the same phage proceeded to much lower values. This experiment, there- 

 fore, in automatically selecting for phage with intact B-cistron function, 

 simultaneously selects for particles which are free of hits in the immediate 

 vicinity of the marker but have suffered typically in other regions of the 

 genome. Thus, the rll cistrons themselves (as opposed, for instance, to some 

 product of the rll cistrons brought in by the infecting phage) must be hit 

 to inactivate the function of the wild-type allele. 



Other experiments by Krieg (personal communication) show that if an 

 rll wild-type cistron is to function at all in determining growth in K12(A), 

 it must be present in the input phage; i.e., there is no opportimity for it to 

 form by genetic recombination and then express its function. Thus the rll 

 cistrons have many of the properties expected of "vulnerable centers." 

 Since rll mutants, and even chromosomal aberrations which may be dele- 

 tions (Benzer, 1955) involving the rll region grow well in E. coli B (the 

 commonly employed host cell), the rll cistrons almost certainly do not 

 correspond per se to the vulnerable centers revealed by standard multi- 

 plicity reactivation experiments. However, they should reveal themselves 

 in multiplicity reactivation experiments which use K12(A) as host cells. This 

 expectation is borne out by the experiments described below. 



5. Multiplicity Reactivation of Ti in Cells of Escherichia coli K12(A) 



a. The rll Cistrons as Vulnerable Centers. Krieg demonstrated that a wild- 

 type rll cistron in an irradiated particle could perform its function on behalf 

 of an unirradiated rll-mutant particle in the same cell. To qualify fully as 

 vulnerable centers, the rll cistrons in irradiated phage must be shown to be 

 able to perform fimctions which lead to the production of phage in the 

 presence of only other irradiated phage. Epstein (1958) has demonstrated that 

 the rll cistrons do behave in this fashion. He compared multiplicity reactiva- 

 tion in the following two experiments: (1) Wild- type T4 was irradiated and 



