364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



It was found that genetic factors could be carried from one type of 

 Salmonella cells to another type by means of a bacterial virus. In 

 this type of transformation the genetic fragment is not free but is 

 carried within the structure of the bacterial virus. It is, for example, 

 not affected by the enzyme deoxyribonuclease, and in this respect is 

 unlike the DNA pneumococcus transforming principle. However, it 

 is not necessary for the virus actually to possess virus activity, for 

 killing of the virus by ultraviolet light does not prevent the transduc- 

 tion of other traits. The closeness of the relationship between the 

 virus and the genes of the host is emphasized by the fact that the 

 transducing ability of any bacterial virus is determined strictly by the 

 character of the cells on which the virus was most recently grown. 

 Virus grown on Serotype E 2 Salmonella cells will, when added to 

 Serotype E x cells, convert a fraction of these cells into Serotype E 2 

 cells. It is of interest to note that the virus in filtrates of toxin-form- 

 ing bacterial strains will convert nontoxin- forming cells into toxin- 

 forming cells. In transduction, a fragment of a chromosome which 

 might be regarded as a gene or a collection of a few or even many genes 

 can be transferred from one kind of donor cell to another kind of 

 receiver cell and be incorporated into the genetic apparatus of the 

 receiver cell. In the pneumococcus or influenza bacterium this can be 

 caused by a DNA preparation which can be separated and isolated as 

 such and in Salmonella this gene or gene collection rides within the 

 bacterial virus, presumably with the viral DNA, which is added to the 

 cell to be transduced. Here one hardly knows what to call a virus and 

 what to call a gene for it is obvious that at times the two merge 

 completely. 



The persistence of a bacterial virus in an apparently concealed 

 form of prophage in lysogenic strains of bacteria, extensively investi- 

 gated by Lwoff, provides further evidence in this direction. Lyso- 

 genic bacteria perpetuate in what may be considered a hereditary 

 manner the property of being able to produce a bacterial virus. The 

 term "prophage" is used to describe the form in which the potentiality 

 to produce a bacterial virus is perpetuated in lysogenic bacteria. 

 Prophage is nonpathogenic and noninfectious in the usual sense, but, 

 since it is multiplied at least once with each cell division, it may be 

 regarded as infectious in the sense that genes or chromosomes are in- 

 fectious. In other words, the prophage might be considered as a 

 temporary part of the genetic apparatus of the cell, the genetic element 

 that differentiates a lysogenic from a sensitive cell, and at the same 

 time as the noninfectious form of a bacterial virus. There are times, 

 therefore, when a virus may not exhibit its normally infectious nature 

 but have its potentially unlimited reproductive capacity under genetic 

 control so that it replicates only once with each cell division. There 



