RECOMBINATION IN VIRUSES AND BACTERIA 131 



presumablv by mutation from the defective state, can transduce the gal 

 marker with an efficiency approaching 100 per cent. This is probably 

 due to the fact that the defective prophage and the attached gal locus 

 form a tightly integrated unit loosely attached to the genotype of the 

 heterogenote. 



During the multiplication that follows the usual invasion by an exo- 

 genous phage, there is only a small chance for any locus from the dis- 

 integrating bacterial chromosome to become included within a phage and 

 to be transduced. And then again, there is only a small chance that, 

 once carried over, it will be incorporated in the genotype. 



TRANSFORMATION BY DNA 



There is really little difference between transduction and transforma- 

 tion, other than the way the DNA fragment is transferred, by a phage of 

 by an experimenter. In both processes, exogenous DNA is incorporated 

 into the genome of the receptor cell. It seems likely that the mechanism 

 of incorporation is similar in the two systems, perhaps like that presented 

 in Figure 5.9, but there is little experimental evidence on this question 

 as yet. Of course, in transduction the bit of DNA in the phage coat is 

 protected from the destructive action of the enzyme, deoxyribonuclease 

 (DNA-ase), and the extracted fragment in transformation must find its 

 own way into the cell. The latter may be achieved through special re- 

 ceptor sites on the host cell surface. We suppose this as a result of ex- 

 periments in which a nontransforming DNA is used as an inhibitor. If 

 the competition is for adsorption, then the nontransforming DNA will 

 depress equally both the uptake of transforming DNA and the number 



TABLE 5.3 



Inhibitron by Nonradioactive, Nontransforming DNA of the Uptake of P^^- 



Labeled Transforming DNA and of the Number of Transformants 



(From Schaeffer, 1958, in Biological Replication of Macroinolecules. 

 New York: Academic Press, p. 60) 



Percentage Inhibition of 

 Concentration of Retention, by DNA-ase Treated 

 Inhibitory DNA Bacteria, of P^^ DNA Transformation 







3 44 42 



10 73 71 



30 87 86 



