366 F. W. STAHL 



for two closely Imked markers; the reactivating phage is mutant for one 

 marker which lies between the other two. A "reactivation" is scored when- 

 ever the wild-type allele at the central locus in the irradiated parent appears 

 in a progeny phage particle along with wild-type alleles at each of the 

 bracketing loci. Within our scheme, the mutant loci in the irradiated parent 

 may be thought of as "ultra\dolet-induced damages," which, even at zero 

 dose, prevent reactivation in a fraction of the cases. In order for the reactiva- 

 tion to be rendered less likely by radiation, a damage must occur between 

 the two mutant loci. Thus, the fraction of infected cells producing wild-type 

 phage should fall below the curve obtained with a single marker until a dose 

 is reached which assures that most of the phage have been hit between the 

 two mutant loci. Then the curves should become identical. In both the above 

 experiments, the predictions of the hypothesis of rescue by recombination 

 are borne out in detail (Doermann and Chase, personal communication). 



d. Quantitative Aspects. The evidence presented above indicates that some, 

 at least, of the ultraviolet light-inactivation is due to hits on the genetic 

 structure as we know it from mapping experiments (see Chapter VIII, 

 Volume II). The same experiments permit estimates of the fraction of the 

 total hits which fall into this category. One estimate can be derived from the 

 initial slope of a cross-reactivation survival curve, another from the incre- 

 ment added to the final slope when two closely linked markers are used in- 

 stead of a single marker. 



To estimate the fraction of hits on the genome from an examination of the 

 initial portion of a marker survival curve, we need values for: (1) the slope 

 of the curve; (2) the map distance between two markers which are just far 

 enough apart to be essentially independently knocked out at low dose; and 

 (3) the total map length of the T4 genome. The experiments of Doermann 

 et al. give the first two values as 0.06 and 12-20 (we shall use 16), respectively. 

 The total map length is not accurately known but is probably at least 200 

 units. The total genome can accommodate 200/16 =13 independently 

 inactivated regions. Since each of these regions has a sensitivity of about 

 0.06, we conclude that 0.06 X 13 = 0.8 of the ultraviolet hits are accounted 

 for by low-dose cross-reactivation experiments. 



To estimate the fraction of hits on the genome from high-dose data, we 

 need values for: (1) the increase in ultimate slope arising from the require- 

 ment of a two-marker reactivation, along with (2) the map distance between 

 the two factors, and (3) the total map length. For the marker pair, ^205—^320^ 

 for instance, the first two values are 0.0021 and 0.47, respectively. Thus, 

 ultraviolet light delivers 0.0021/0.47 = 0.0045 hits per map unit per phage- 

 lethal hit. Again assuming a map length of 200 units, we calculate that 

 0.0045 X 200 = 0.9 of the ultraviolet hits to the phage are accomited for 

 by the high-dose experiments. 



