370 F. W. STAHL 



reaction is measured in K12(A). In mixed infection of K12(A) by an rllA and 

 an rllB mutant, only those cells which are infected by at least one of each 

 type of mutant produce phage. If the two types of phage are adsorbed to 

 K12(A) at low (cir, 0.1) multipHcity of each type, the majority of cells 

 producing phage do so by virtue of having been infected by exactly one of 

 each type; a calculable number contain two of one type and one of the other; 

 an insignificant proportion contains two of each type. The theoretical sur- 

 vivals for the mixed doubly and triply infected cells are obtained by simple 

 modification of Equation (5). We assume that the average sensitivity for 

 the function of an rll cistron is 0.075 of the total sensitivity and that the 

 three vulnerable centers which act in E. coli B as well as in K12(A) comprise 

 0.40 of the total target. Figure 8 compares Epstein's (personal communica- 

 tion) data with the curve generated by these considerations with encouraging 

 results. 



b. Genetic Recombination During Multiplicity Reactivation. In addition to 

 being of different genotype in the rll region, the input phage in a low- 



■ Genetic structure of T4 (ca. 200 map units)- 

 rH region 



A B 

 cistrons Unidentified vulnerable centers 



Fig. 9. The structure of T4 as revealed by studies on ultraviolet-inactivated phage. 

 The primary target for ultraviolet light is the linear genetic structure. Three regions 

 (shaded) of the genome perform functions necessary for duplication and recombination. 

 The A and B cistrons of the rll region (cross-hatched) have such properties when T4 

 is examined in the host K12(A). 



multipHcity experiment in K12(A) may be made different at other loci whose 

 functional survival does not determine the ability to multiply. In these 

 experiments, the frequency of phage recombinant for these nonselective 

 markers increases with dose of ultraviolet light dehvered to the parents 

 (Epstein, 1958). Furthermore, at high doses, a number of yields from single 

 bacterial cells are composed over 90 % of one genotype which is often one 

 of the recombinant genotypes. This is the result demanded by the hypo- 

 thesis that recombination between the input phages reconstitutes a genome 

 free of damage which then multiplies to become the predominant type in 

 the cell. The other genotypes released in smaU numbers in any given cell 

 arise by cross-reacting interactions between this viable multiplymg popula- 

 tion and the inactive particles in the cell. 



On the basis of the experiments described, we can construct a formal 

 picture of T4 as it appears to a photon of ultraviolet hght (Fig. 9). 



