BACTERIOPHAGE I3 



pletely predominant one, in what could be called parasitism at the 

 genetic level. In the so-called "lysogenic" strains of bacteria, which 

 carry and occasionally liberate phage (29), the genetic patterns of 

 host and virus may coexist and function side by side in genetic sym- 

 biosis. The difference between phage infection followed by death of 

 the host and phage infection followed by lysogenicity may thus be 

 interpreted as a difference in compatibility relations between the ge- 

 netic materials of virus and host. The compatibility in lysogenic systems 

 may be more or less stable, and its changes may be connected with 

 the sporadic character of phage liberation by lysogenic bacteria (29). 



Stretching the available evidence, one may construct the following 

 picture of the reproduction of a bacteriophage such as T2: Infection 

 produces a disruption of the genetic organization of the host and a 

 change in the organization of the infecting virus leading to the forma- 

 tion of a new unit system, the virus -infected cell, containing the exist- 

 ing enzymatic machinery of the host and, superimposed upon it, a 

 genetic pattern derived from the virus and directing the synthesis of 

 virus material from non-specific building blocks. This genetic pattern 

 is resolved into a mmiber of discrete, more or less independent units, 

 the genetic determinants of the virus. The process of formation of the 

 new virus is such that it allows for complex reorganizations to take 

 place and results in the appearance of a population of virus particles 

 which represent the end products of the process as a whole. 



This picture, which admittedly has a heuristic rather than de- 

 scriptive function, presents several major gaps. First, there is a time 

 gap between the disappearance of the initial virus and the appearance 

 of the mature virus. Second, there is a chemical gap between the 

 aspecific building blocks and the final specific nucleoprotein particles. 

 Third, we have a genetic gap between the determinants of the phage 

 and the phage particle, the former being responsible for the inherited 

 specificities, the latter being the carrier of infectivity and, therefore, 

 the only operationally definable unit in the extracellular state. Finally 

 we have a technological gap, in our ignorance of the enzymatic ma- 

 chinery involved in the synthesis of phage from the newly assimilated 

 building blocks. I do not emphasize these gaps in a spirit of pessimism, 

 since it is clear than they involve phases of biological replication about 

 which no biologist possesses any information. The very fact that these 

 gaps can clearly be visualized and delimited in phage analysis suggests 

 that they may be filled more easily by work on bacteriophage than on 

 other biological systems. 



