ZINDER AND LEDERBERG 



Table 1 



Mutant strains mid symbols used 



223 



prototrophs so infrequently and spo- 

 radically as to be of doubtful signifi- 

 cance. It has since become evident that 

 LA-2 2 is genetically a single, stable 

 mutant although it was derived in two 

 steps and has a complex nutrition. 



LT-22 is lysogenic for a virus (here- 

 after referred to as PLT-22) active on 

 LT-2. This virus is capable of inducing 

 lysogenicity in LT-2. Among the lyso- 

 genic derivatives of LA-2 three differ- 

 ent interaction groups were found: the 

 majority no longer interacted with 

 LA-2 2 to give prototrophs; a few in- 

 teracted with impaired efficiency; still 

 fewer were not affected in this respect. 

 These experiments indicated that ge- 

 netic exchanges did occur and that 

 latent bacteriophage played some role 

 in the interaction. 



Indirect crosses: platings of cells and 

 filtrates. To test the possible role of 

 filtrable factors in this interaction, a 

 U-tube with an "ultra-fine" sintered 



pyrex filter partition was prepared ac- 

 cording to Davis's (I95OZ7) design. By 

 alternating suction between the arms 

 of the tube, two intact populations of 

 growing bacteria could be made to 

 share the same medium. The integrity 

 of the filter was verified in control 

 experiments by leaving one compart- 

 ment uninoculated. Then 10^ cells of 

 each parent were inoculated into 

 twenty ml of broth and placed in 

 either arm of the tube. Ten ml were 

 flushed from side to side every twenty 

 minutes for four hours while the cul- 

 ture grew to saturation. The two pop- 

 ulations were washed and plated upon 

 minimal medium. Prototrophs ap- 

 peared in the platings of LA-22 but 

 not of LA-2. Sterile filtrates of LA-2 

 broth cultures did not elicit proto- 

 trophs from LA-22. However, filtrates 

 from mixed cultures of LA-2 and 

 LA-22 elicited about one prototroph 

 per million LA-22 cells. Thus LA-2 



