Transduction 



331 



trates of the M~T+ strain on M + T~ cells 

 — does not produce recombinants. The 

 two strains differ, therefore, in donor ca- 

 pacity. 



The M + T~ donor strain (but not the 

 M~T + strain) is found to harbor a phage. 

 This virus, P22, is said to be nonvirulent 

 or temperate. Nevertheless, about one in 

 a thousand times this phage replicates and 

 lyses or bursts the host cell, liberating up 

 to several hundred progeny phage. Ac- 

 cordingly, a culture of bacteria harboring 

 temperate phage does not show a conspicu- 

 ous amount of lysis. Because each cell of 

 the M + T~ strain carrying P22 is poten- 

 tially subject to lysis, the strain is said to 

 be lysogenic. (The lysogenic bacterium, or 

 lysogen, is immune to new infection — that 

 is, to superinfection — by identical or ho- 

 mologous phage.) On the other hand, the 

 M~T + strain normally lacks P22 and is a 

 nonlysogenic or sensitive strain. When a 

 sensitive strain is exposed to temperate 

 phage, a relatively large fraction of the 

 newly-infected cells lyse and liberate phage. 

 But a small fraction is able to survive, be- 

 come lysogenic, and give rise to lysogenic 

 progeny. If lysogens are lysed artificially 

 and tested for phage, none are detected. 

 Apparently, the phage in a lysogen is con- 

 verted to a new form, called prophage, 

 which reproduces at the same rate as the 

 host chromosome. Usually, to lyse a lyso- 

 gen, prophage must first rapidly replicate a 

 number of times to produce the infective 

 phage liberated at the time of lysis. 



What is the relationship between the fil- 

 terable M+ factor and the phage P22? 



1. Both are unaffected by RNase and 

 DNase. 



2. Both show the same inactivation pat- 

 tern with temperature changes. 



3. Both have the same susceptibility to 

 an antiserum that blocks the attachment of 

 phage to the bacterium. 



4. Both become attached to susceptible 

 cells simultaneously. 



5. Both have the same size and mass as 

 determined by filtration and sedimentation 

 tests. 



6. Both appear in the medium at the 

 same time and in a constant ratio. 



7. Both retain this ratio even though var- 

 ious purification and concentration proce- 

 dures are applied. 



From these results, it is evident that M + 

 is associated with the phage. Since the ge- 

 netic material of Salmonella is known to be 

 composed of DNA, it is likely that the ge- 

 netic factor M+ is also composed of DNA. 

 Moreover, because the M+ genetic factor 

 cannot be located on the outer surface of 

 the phage particle, the M+ gene must be 

 located in the interior of the virus. 



Genetic transduction is the process of ge- 

 netic recombination made possible by a vi- 

 rus particle introducing homologous DNA 

 into a recipient cell. 



Are there any restrictions on the genetic 

 material of Salmonella which can be trans- 

 duced by P22? This virus can be grown 

 on sensitive bacteria genetically marked 

 M + T + X+Y~Z~\ the crop of phage pro- 

 duced after this infection can be harvested, 

 and a portion tested on sensitive indicator 

 strains (M~, T~ , X~, Y~, Z~) one at a 

 time. The results of such tests show trans- 

 duction of M + , of T+, and of X + — but not 

 of Y + or Z + . Another portion of the har- 

 vested phage is grown on another genetically- 

 marked, sensitive strain — M + T~X+Y + Z-, 

 for example. When the new phage crop is 

 harvested and then tested on the indicator 

 strains already mentioned, it is found now 

 that the new crop of phage has lost T+ but 

 has gained Y + transducing ability. These 

 results demonstrate that a phage filtrate has 

 a range of transduceable markers exactly 

 equal to that of the markers present in the 

 bacteria on which the phage was last grown. 



