On the Nature of Viruses, Genes and Life 319 



readily be considered an infectious process, and that viruses can act as genes 

 and genes as viruses under certain circumstances. 



This information as well as our knowledge of viruses was soon fortified by 

 the very important discovery by Zinder & Lederberg in 1952 of transduction in 

 Salmonella by means of a bacterial virus. 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 imlike 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 transduction 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 E2 Salmonella cells will, when added to Serotype Ei cells, convert 

 a fraction of these cells into Serotype E2 cells. It is of interest to note that the 

 virus in filtrates of toxin-forming bacterial strains will convert non-toxin-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 of 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. 



I hope that by this time it is obvious that viruses, genes and life are tied to- 

 gether by a series of relationships, that viruses can act as genes and genes as 

 viruses under certain circumstances, and that viruses are structures at the 

 twilight zone of life partaking both of Hving and of molecular properties. Let 

 us now see if there is a common thread of understanding permeating all of these 

 relationships. We know that viruses have been thought to be at least as complex 

 as a nucleoprotein, but we also know that the transforming agent of the pneu- 

 mococcus has been found to be a deoxyribonucleic acid and there is presumptive 

 evidence that the genetic stuff" of the bacterial viruses is also deoxyribonucleic 

 acid. However, until recently no gene or chromosome or any of the ordinary 

 viruses had been isolated as such in the form of nucleic acid, hence the 'stuff 

 of life', as well as the viruses, has been considered to be nucleoprotein in nature 

 with considerable doubt as to whether the protein or the nucleic acid or the 

 combination of the two was really the biologically active structure. 



A recent very important discovery made in our laboratory by Dr. Fraenkel- 

 Conrat has changed the situation considerably and now makes it seem certain 

 that nucleic acid is the all-important structure. It was reported by Fraenkel- 

 Conrat and also shortly thereafter by Gierer & Schramm in Germany that 

 special treatment of tobacco mosaic virus yielded a nucleic acid preparation 



