THE INITIATION OF BACTERIOPHAGE INFECTION 223 



discussed in Section IV, there are particles possessing the host-range pheno- 

 type of one of the parents and the host-range genotype of the other parent, 

 indicating that phage tails (determining host-range phenotype) and DNA 

 (determining genotype) are paired unspecifically from components derived 

 from both parents during formation of phage progeny. Similar results have 

 also been obtained for the tryptophan-dependence characteristic in crosses 

 between dependent and independent strains (Brenner, 1957). Second, there 

 are particles possessing the host-range phenotype (but not the genotype) of 

 both parents (Streisinger, 1956), as recognized by their capacity to attach 

 both to B/2 and B/4 cells. The existence of these phenotypicaUy hybrid 

 particles shows that at least two kinds of attachment sites can be present in 

 a single particle (perhaps located on separate tail fibers) and that each site 

 can function independently. 



The relationship of tryptophan dependence to host range has been studied 

 by crossing strains of T2 and T4 having different tryptophan requirements 

 (Bremier, 1957). Phenotypic analysis of the immediate progeny from the 

 crosses showed that the host-range and tryptophan-dependence character- 

 istics were paired almost at random in an individual particle; that is, a par- 

 ticle with the host-range phenotype of one of the parents had almost a random 

 chance of having the tryptophan-requirement phenotype of the other parent. 

 However, these two characteristics were found to be genetically linked; that 

 is, a particle with the host-range genotype of one of the parents was likely to 

 have also the tryptophan-dependence genotype of that parent. These results 

 indicate that tryptophan-dependence and host-range phenotypes are deter- 

 mined by separate components of the phage coat (corresponding to the 

 separate regions of the chromosome that determine the two genotypes, as 

 discussed above). 



VII. On the Mechanism of Invasion 

 A. Alterations of Phage Tail Structure during Invasion 



1. Morphological Changes 



In 1955, Kellenberger and Arber published electron micrographs (Fig. 8) 

 showing the changes in intact T2 (and T4) resulting from interaction with 

 cell walls prepared by the procedure of Weidel (1951). It can be seen that a 

 radical alteration of the tail structure has occurred. The fibers at the distal 

 portion of the tail are no longer visible, and there is revealed the core of the 

 tail. The proximal portion of the tail is now about 40 % thicker than in intact 

 particles. Essentially the same alterations in the phage tail after interaction 

 with cell walls have since been observed by Brown and Kozloff (1957). In 

 Fig. 8 it should be noted that, although the tails are altered, the heads 



