228 



total of eight unselected markers there 

 was no evidence of co-segregation. 

 These experiments were repeated with 

 active filtrates from LT-2 and gave the 

 same result. 



Genetic transfers for each of three 

 markers (one nutritional and two fer- 

 mentative) were observed when ex- 

 periments were set up in such a way as 

 to select for them. SW-435 (Aux, 

 Gal—, Xyl— , S"") was plated with FA 

 (from LT-2 Prot, Gal + , Xyl + , S«) on 

 minimal, EMB galactose and EA4B 

 xylose agar. Upon the EMB media 



ZINDER AND LEDERBERG 



there first appeared a thin film of 

 growth (pink and hence nonferment- 

 ing) and then small outcroppings or 

 papillae which fermented the galactose 

 or xylose. These papillae grow quite 

 large (figures 3 and 4) because of their 

 utilization of the sugar when other nu- 

 trients are depleted. The xylose-nega- 

 tive mutant gave some papillae due to 

 spontaneous reversion but not enough 

 to interfere with the scoring of the 

 test. The galactose negative mutant is 

 more stable and has only rarely re- 

 verted. The number of papillae on 



.-•■.•. ^ 



•• • • • • • 



• • • • 



Figs. 3 and 4. SW-435 plated on EMB galactose agar with heat inactivated (3) and 

 active (4) FA. 



EMB or prototrophs on minimal agar for different selected characters may 

 (table 2) was approximately the same be uniform. However, the unselected 

 so that the efficiency of transduction markers remained unaltered; that is, all 



Table 2 

 SW-435 and FA Jipon different selective media 



Figures are colonies or papillae per plate. 



