RECOMBINATION IN VIRUSES AND BACTERIA 



127 



phaneless strain will not transduce tryptophaneless bacteria of the same 

 strain which are, of course, mutant at the same site. To succeed, the 

 phages must be grown on tryptophane-synthesizing bacteria, or at least 

 on bacteria that have the wild-type allele of the locus that is mutant in 

 the recipient strain. A suspension of sensitive bacteria infected by 

 transducing phages may simultaneously be transduced and undergo lyso- 

 genic conversion. But the two processes may also occur independently 

 in the same suspension; some bacteria may be lysogenized and not trans- 

 duced — they are converted. Others may be transduced and not con- 

 verted. In this case the infection is initiated and the contents of the 

 phage coat are injected but, for some reason, the phage DNA aborts 

 and neither enters the productive cycle nor becomes reduced to pro- 

 phage. Only the bacterial DNA is incorporated and assumes a function. 

 Sometimes a phage may even carry from cell to cell the prophage of 



"- "^ 



Phages were grown on 



mef^ gal"^ xyl"^ str-r- 



bacteria 



•-Za^ 



Recipient bacteria are 

 "met ~ gal ~ xyl ~ str-s 



v_/ 



Glucose 



Galactose 

 methionine 



Xylose 

 methionine 



Glucose 

 methionine 

 streptomycin 

 met ■*■ gal ~ xyl ~ str-s met ~ gal ~ xyl * str-s 



met ~ gal '*' xyl ~ str-s m£t ~ gal ~ xyl ~ str-r 



FIGURE 5.8. Transduction of one character at a time in Salmonella typhimurium 

 (after Zinder and Lederberg, 1952, J. Bacf., 65:679). 



