230 



with each of the three auxotrophs in 

 similar experiments. In the course of 

 transduction, there was no Hnked seg- 

 regation or association of these three 

 nutritional markers with each other or 

 with fermentative markers. Strepto- 

 mycin resistance provided still an- 

 other marker that remained unaltered 

 in cells transduced for other char- 

 acters. 



Several galactose-negative mutants 

 were transducible to galactose-positive 



ZINDER AND LEDERBERG 



by FA from their parental wild type. 

 FA from these mutants gave diverse 

 results. The mutants were never trans- 

 duced by their own FA, but they could 

 be transduced by FA from some of the 

 other mutants. These interactions pro- 

 vided a basis for grouping the mutants 

 with respect to allelism or genie iden- 

 tity (table 4). 



All of the transductions discussed 

 thus far have been in the direction of 

 mutant to wild type. It is difficult, as 



Table 4 



The effect of FA fro?n several galactose-negative umtayits upon these sajne jmitants 



* Galactose positive papillae produced. 

 t No more papillae than on control. 



a rule, to screen for changes in the 

 other direction owing to the lack of 

 adequate selective methods. This can 

 be done with streptomycin resistance 

 since the wild type condition is sensi- 

 tive (S^) and the mutant is resistant 

 (SO- Freshly harvested cells were ex- 

 posed to FA from streptomycin re- 

 sistant and sensitive "parents" and then 

 plated upon EMB galactose. After two 

 hours of incubation (to allow for phe- 

 nomic lag, Davis 1950^) the plates 



w^ere sprayed with a concentrated so- 

 lution of streptomycin (0.1 g per ml). 

 Table 5 shows that the transduction 

 did occur but only when a strepto- 

 mycin resistant mutant was the source 

 of the FA employed. No associated 

 changes were found. The stability of 

 the transduced cells was verified by 

 tests of many daughter colonies by 

 replica plating (Lederberg and Leder- 

 berg, 1952) to normal and strepto- 

 mycin containing media. 



Table 5 



Compariso?? of the effect of FA from streptomycin resistant and 

 sensitive cells on sensitive cells 



Figures are the number of streptomycin-resistant colonies per plate. 



