LETHAL ACTION OF X-RAYS IN E. coli K12 



177 



strain. On the other hand, the difference between the slopes of the two 

 curves may vary sUghtly from one ex])eriment to another. 



2. Survival CKires of T+ Sr, L+ Sr and T+ L+ Sr recombinants 



If the decrease of the frequency of the recombinants with increasing 

 doses of X-rays was related to inactivation of the selected marker, as 

 for instance, mutation from prototrophy to auxotrophy for the con- 

 trolled substance, the slope of the T+ L+ Sr recombinant survival curve 

 would be the sum of the slopes of the T+ Sr and of the L+ Sr survival 

 curves, assuming that the inactivation of the T and of the L markers 

 are independent events. Figure 2 shows that this is not the case. The 



Fig. 2. — The same conditions as for Fig. 1. The crosses are plated on synthetic agar 

 without T and L or with one of each amino acid in order to select for one marker only, 



or for both. 



slojDcs of the three survival curves are very close : hence the transfer of 

 the two T L markers to the recombinants and its inactivation depend 

 on closely -linked events. This may be demonstrated directly by the 

 genetic analysis of the surviving T+ Sr and L+ Sr recombinants. Nearly 

 100 per cent of them are found to carry the non-selected marker re- 

 spectively L+ and T+. This result indicates that the loss of the recom- 

 binants from an X-rayed Hfr is linked to some damage to the chromo- 

 some, apart from the selected markers themselves. 



