SYLLABUS 1 35 



(a) Luria discovered another kind of reactivation of UV phage. 

 It occurs when one bacterium is infected with two or more inactive 

 particles. This is called multiplicity reactivation (MR). The amount 

 of MR is defined as the fraction of the multiple-infected bacteria that 

 liberate active phage. This fraction may be close to unity. The amount 

 of MR decreases with increasing UV dose. It increases with increasing 

 multiplicity. 



Luria has proposed a theory to explain MR. According to this 

 theory a phage particle consists of a number of specific units, the pres- 

 ence of one unit of each type being necessary for the formation of an 

 active phage particle. The units are independently hit by UV radiation 

 and one hit in one unit is sufficient for inactivating a phage particle. 

 It is assumed that a phage particle after adsorption on the sensitive 

 bacterium breaks up into the elementary units which multiply inde- 

 pendently of each other in the first part of the latent period, and re- 

 assemble later, constituting the phage particles which will appear in 

 the yield. If more than one inactive particle is adsorbed by the same 

 bacterium, production of active phage will take place in those bacteria 

 in which one copy at least of each unit has not been hit by the radia- 

 tion. Quantitative formulation of this hypothesis leads to expectations 

 which are in partial agreement with the experimental data: the devia- 

 tions of the expected from the observed results can be accounted for 

 by several possible complications. 



(b) Mixed infection with two inactive particles marked at one 

 genetic locus (Luria and Dulbecco, 1949). 



In a population of bacteria mixedly infected with a very low multi- 

 plicity ( < < 1 ) of inactive T2r+ and inactive Tar most of the mixedly 

 infected bacteria are infected with only one T2r+ and one T2r particle, 

 both inactive; under such conditions most of the bursts resulting from 

 mixedly infected bacteria (due to MR) should contain T2r+ together 

 with the Tar particles. This is actually what is found. A few of the 

 bursts are not mixed, and this is explained by the units theory by con- 

 sidering that in some of the infecting particles the particular unit 

 carrying the r locus was hit by the radiation. 



(c) Mixed infection with one active and one inactive particle 

 marked at one genetic locus (Dulbecco and Luria, unpublished). 



With a technique similar to that mentioned in the previous section 

 it is possible to study the yield from bacteria infected with one active 

 T2r+ and one inactive T2r particle, and vice versa. The fraction of 

 bursts in which the allele of the r locus of the irradiated type is not 



