ZINDER AND LEDERBERG 



231 



It is now evident that the particular 

 FA for which an assay has been de- 

 fined is just one of several coexisting 

 functions of a given filtrate. We are 

 entitled to refer to FA for any of the 

 genetic factors so far studied, and the 

 range of action of a given filtrate can 

 be designated in the same way as the 

 genotype of the culture from which 

 it is obtained: e.g., Prot, Gal-|-, Xyl + , 

 S' for SW-514 (figure 5), as we'll as 

 for the FA derived from it. Unless 

 otherwise qualified, however, FA will 

 continue to refer to the transduction 

 assayed on LA-22. 



Adsorption of FA. The first step in 

 transduction must be the adsorption of 

 FA on competent cells. LA-22 was 

 harvested from nutrient agar plates. 

 Aliquots were suspended in one ml of 

 an active filtrate for various intervals. 

 The cells were sedimented and plated 

 on minimal agar to determine the num- 



ber of exchanges. After a heat shock 

 at 56 C to destroy any unsedimented 

 cells, the supernatants were assayed 

 with LA-22 for unadsorbed FA. A4od- 

 erate amounts of FA were completely 

 adsorbed within the time necessary for 

 centrifugation (15 minutes) and were 

 recovered quantitatively in the pre- 

 cipitated cells. 



All tested smooth strains of S. 

 typhiiimrhmi adsorbed FA. Cells of 

 the donor strain adsorbed as efficiently 

 as the others, consistently with the 

 success of intrastrain transfers. Disin- 

 fection b\' boiling or ultraviolet ir- 

 radiation (to leave an extremely small 

 viable fraction) did not aff^ect adsorp- 

 tion. Rough cultures, selected by ag- 

 ing in broth (Page et al., 1951) did not 

 adsorb. These results indicated that the 

 site of adsorption is heat stable, is not 

 afi^ected by the death of the cell, and 

 may be related to the somatic antigen. 



FA (SW-5l4i PROT, GAL + .XYL + .S'') 



ON 



SW-351 (AUX,GAL-,XYL-,SS) 



MINIMAL 



EMB GAL 



EMB XYL STREPTOMYCIN 



Fig. 5. Multiple potentialities of an active filtrate. 



With the amounts previously used, 

 FA assays were directly proportional 

 to FA concentration. Cells of LA-22 

 were harvested from nutrient agar. 

 Aliquots containing 10^" cells were 

 sedimented in each of ten centrifuge 



D 



tubes and the supernates discarded. 

 Multiple aliquots of FA (one to ten 

 ml) were added and 15 minutes at 37 



C allowed for adsorption. Supernates 

 and cells were collected and assayed 

 on EiML galactose. No concordant 

 changes (i.e., galactose positive) were 

 observed among the prototrophs. Fig- 

 ure 6 shows that a maximum number 

 of transductions occurred with about 

 eight ml of FA. The saturated sedi- 

 ments adsorb no more FA from larger 



