GENETICS OF BACTERIOPHAGE 



TABLE 1. TRIPARENTAL RECOMBINATION TESTS 



WITH THREE INDEPENDENTLY ASSORTING 



GENETIC FACTORS 



Results are shown for two independent experiments of each kind. 



antiphage serum just sufficient to reduce the titer of the mixture by a factor 2 to 

 4 after equihbrium has been reached. This requires several hours at room temper- 

 ature. The mixture now yields, on plating, about two per cent of mottled 

 plaques. 



Mottled plaques coming from clumps containing the doubly marked phages hrl 

 and wild-type are found, on sampling, to contain the two parental types of phage, 

 together with the expected recombinants. The same is true of mottled plaques 

 originating from bacteria infected with hrl and wild-type. This shows that there 

 is no strong selection among the several genetic types of phage during plaque for- 

 mation. It shows, incidentally, that the genetic result of a mixed infection is of 

 the same kind whether the parental phage particles attach to the bacterium at the 

 same pomt, as with mottling clumps, or at different points, as m the usual mixed 

 infection. 



The mottling clumps are identifiable as clumps by their abnormal sensitivity to 

 antiserum. When 90 per cent of a population containiiig two per cent of mottling 

 clumps is inactivated by exposure (at low concentration, to avoid further aggre- 

 gation) to antiserum, the proportion of mottling clumps among the survivors is 

 less than 0.2 per cent. 



When a population of phage containing two per cent of mottling clumps (hrl + 

 wild-type) is adsorbed to B so as to infect one bacterium in ten, and the infected 

 bacteria are plated on B after inactivating the unadsorbed phage with antiserum, 

 two per cent of the plaques from infected bacteria are mottled. The resistance of 

 the adsorbed clumps to antiserum shows that both members of a mottling clump 

 infect the same bacterium. When the same population is adsorbed to B/2, and 

 the infected cells are treated with antiserum or merely washed to remove unad- 

 sorbed phage, only hr plaques are formed on plates seeded with B. Evidently 

 both members of a mottling clump have to make specific attachments to the bac- 

 terium in order to produce a mixed yield. This result probably explains why we 

 are unable to prepare populations contaming more than a small fraction of mot- 

 tling clumps by agglutination with antiserum. Only those clumps in which the 

 two kinds of virus are oriented in a manner favorable to the attachment of both 

 to the same bacterium can produce mottled plaques. 



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