RECOMBINATION IN BACTERIOPHAGE 65 



ficult to reconcile, in terms of a single mechanism, with the existence of a 

 second class of crosses yielding about 15 percent of wild type. 



Second, the correlation between proportions of the two recombinants in the 

 cross hXr7, and the lack of a corresponding correlation in the cross hXrl, is 

 incompatible with a single mechanism for the two crosses. It must be recalled, 

 however, that the correlations found are too weak to be wholly convincing. 



Third, Luria's (1947) evidence for a mechanism of independent multiplica- 

 tion and transfer of subunits of the virus, and ours for a system of linkage, re- 

 quire dissimilar types of interpretation. 



The model to which these considerations seem to lead is described below, 

 but we do not consider that we have decisive answers to any of the questions 

 originally posed. The remainder of this discussion is of value only insofar as 

 it clarifies the questions, and systematizes the experimental results so far ob- 

 tained. 



The two linkage structures bearing the markers rl and h, respectively, are 

 assumed to be examples of the class of independently multiplying subunits of 

 the virus whose minimal number Luria and Dulbecco (1949) estimate at 

 about 25. The reconstitution of virus from these units must be regulated in 

 such a way that each particle receives one representative of each kind of unit. 

 In the cross hXrl the choice between h and h + , and between r and r + , is de- 

 cided nearly at random to yield on the average 37 percent of recombinants and 

 63 percent of the parental types in bacteria yielding equal numbers of the two 

 parents. The deficit of recombinants below 50 per cent is unexplained, but may 

 be thought of as an effect of incomplete mixing between neighboring clones 

 of multiplying virus in the cell. 



According to this hypothesis one expects from the cross hXrl proportionate 

 yields of the two recombinants in a single burst to be: 



P( * + ' +) = K n^T+nJ fa^rW)) (4> 



p(hr) . J "W V *> -) (5 ) 



\ n(h+) + n(A)/\n(r+) + n(r)/ 



where the expressions on the left refer to proportions of recombinant virus, 

 the corresponding expressions on the right refer to the intra cellular yields of 

 the respective unit linkage structures, and the coefficient m expresses the 

 fraction of the intracellular virus which may be regarded as a random mixture 

 of the two parental types, the remainder being considered unmixed. If the 

 structures carrying the markers h, h + , r, and r + grow independently in the 

 cell, their yields will fluctuate independently, and no correlation will be ex- 

 pected between the numbers of the two recombinants in sufficiently small 

 yields of virus. This expectation is borne out by the data for viral yields from 

 single bacteria. The average yields of the two recombinants are equal, however, 

 showing that the several unit structures grow at equal rates. According to 

 equations (4) and (5), the proportionate yield of recombinants should not be 



259 



