362 F. W. STAHL 



Within the framework of the hypothesis for multiplicity reactivation 

 stated above, a cross-reactivation experiment has the following feature: 

 since only one of the input phages has been irradiated, all of the fimctions 

 necessary for phage production are represented in the infected cell; the sur- 

 vival curves for the contribution of genetic markers, then, whatever their 

 basis, are not influenced by the failure of any of the three %Tihierable centers 

 to perform its function. We inquire into the origin of the marker survival 

 curves by posing two questions: (1) How does ultra \'iolet light block the 

 contribution of a genetic marker ("knock out" the marker)? (2) For those 

 markers which are contributed, what is the mechanism by which the 

 marker does appear in the progeny (is "rescued")? The work of Doermann 

 and his collaborators has given satisfying answers to both of these 

 questions. 



b. The Localized Nature of Ultraviolet- 1 7iduced Damage. Doermann et al. 

 (1955) executed a series of experiments well-suited to the detection of 

 discrete lesions on the genetic structure of T4 if such were to exist. They 

 employed as irradiated parent and reactivating parent, respectively, wild- 

 type and triply mutant T4. The wild-type phage was adsorbed at low multi- 

 plicity (e.g., 0.2 particles per cell) to bacterial cells after various doses of 

 ultraviolet light. The triple mutants were adsorbed simultaneously at a 

 multiplicity just sufficient to infect almost all of the cells (e.g., 2-3 particles 

 per cell). The infected cells were highly diluted and distributed into a large 

 number of tubes such that only about 0.2 of the tubes received a cell within 

 which a genetic contribution was made by an irradiated phage. After the 

 cells had lysed, the contents of the tubes were assayed by the plaque assay 

 method. Those plates which showed markers from the irradiated parent 

 were then scored with respect to the eight possible genotypes of phage 

 particles. All the data to be analyzed came from within this class of plates, 

 so that conclusions drawn are based only on observation of irradiated phage 

 which were competent to enter into the events leading to cross reactivation. 

 The following conclusions obtain for this class of particles: (1) Markers 

 which are unlinked in ordinary phage crosses (see Chapter VIII, Volume II) 

 are independently "knocked out" by the ultraviolet irradiation. The markers 

 employed showed approximately equal probability of knock out. If we define 

 J) as the probabihty that a given marker from an irradiated phage particle 

 will appear in at least one copy in the progeny, then we can write the 

 expected frequency of phage contributing 1, 2, or all 3 markers on the 

 assumption of independent knockout. They are 



P, = f{\ - p)l[l - (1 - pf] (9) 



Pz = ^V[l - (1 - Vfl 



