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F. W. STAHL 



survival curve for a marker gives for each dose the probability that the 

 marker will be recombined away from the nearest ultraviolet-induced 

 damage on either side of it in the linear genetic structure. To examine the 

 consequences of this notion we consider the simple case in which the points 

 of recombination between the damaged and nonirradiated phage are not 

 influenced by the position of the ultraviolet-induced lesions. Some phage in 

 the irradiated suspension are destined by chance to have points of recom- 

 bination only rather remote from the marker when they are subjected to 

 the conditions of cross reactivation; others are destined to have points of 

 recombination rather close to the marker. The marker in the first phage will 

 be "sensitive"; that in the second phage will be relatively "resistant." The 



200 400 600 800 



Phage-lethal UV-hits 



Fig. 6. Cross reactivation of a genetic marker of T4 at high doses of ultraviolet light. 

 A culture of Escherichia coli B was infected with about two wild-type particles and two 

 to three mutant particles. The fraction of cells liberating at least one wild-type particle 

 at various doses of ultraviolet light administered to the wild-type parent was then 

 determined. From this value, the fraction of parental irradiated particles contributing 

 the wild-type marker was calculated for each dose. These values are plotted here logar- 

 ithmically versus dose in phage-lethal hits. The figure was composed from unpublished 

 data of Dr. A. H. Doermann and Mrs. M. Chase Epstein for the r^y+ marker of T4. 



These data indicate the knockout of a markei as a result of any hits which influence 

 the ability of the particle to make a genetic contribution. The close similarity in the low 

 dose slope of this curve and the slope of the curve (Fig. 5) for marker survival influenced 

 only by hits on the genetic structure indicates that few, if any, hits prevent a particle 

 from participating in cross reactivation. The decrease in slope with increasing dose 

 agrees with the notion that marker rescue occurs by recombination between an irradi- 

 ated and a live phage. The sensitivity of the r^^+ marker at high dose is somewhat greater 

 than the sensitivity of a number of other markers as determined in similar experiments. 

 The diflerence in sensitivity is probably due to the fact that r^^ is a "chromosomal 

 aberration," whereas the other markers are "point mutations." 



