556 S. E. LURIA 



genetic material. Two consequences follow: First, like all genes, the phage 

 DNA may be expected to control other cellular functions besides its own 

 replication; second, the formation of mature phage may be considered as an 

 expression of the specific genetic function of the phage DNA. Both predic- 

 tions are supported by available evidence. 



1. Conversions of Cellular Properties by Phage 



A number of cell properties that are not obviously related to virus produc- 

 tion are controlled by phage genes. Most remarkable among these is the 

 control of the composition of the cell wall, which manifests itself by specific 

 changes in cellular antigens upon phage infection. This phenomenon has been 

 studied mostly in the genus Salmonella (Iseki and Sakai, 1953; Uetake et al., 

 1955). In what is probably a typical instance, infection with a certain phage 

 results in the appearance, within a few minutes, of somatic antigen 15 and 

 in the equally prompt suppression of the production of antigen 10, This 

 change occurs both in cells in which the phage multiplies vegetatively 

 leading to cell lysis and in cells that survive infection and in which the phage 

 becomes prophage (Uetake et al., 1958). It occurs even in infection with a 

 virulent phage mutant that lyses every infected cell. The reverse change, 

 from antigen 15 to antigen 10, follows the loss or removal of the phage from 

 the carrier cells. 



Clearly, the relation between the phage DNA and the specific constituents 

 of the somatic antigens is no more and no less obvious than the relation 

 between the DNA of a transforming principle and the capsular polysaccha- 

 rides in Pneumococcus (Avery et al., 1944), or, for that matter, than the 

 relation between any gene and the ultimate product of its activity in any cell. 



There is a whole series of these "conversions" of cell properties by phages, 

 ranging from the production of diphtheria toxin (Freeman, 1951) to the 

 abihty to support multiplication of other phages (S. Lederberg, 1957). It was 

 beheved at fiirst that such new properties required the presence of an estab- 

 lished prophage; hence the name of "lysogenic conversions" (J. Lederberg, 

 1955). We reaHze now, however, that these conversions of cell properties are 

 expressions of heterocatalytic activities that may be exerted by all functional 

 states of phage within a cell. 



2. Biosyntheses Related to Phage Replication 



It seems reasonable to attribute to the heterocatalytic functions of phage 

 also the appearance in phage-infected cells of new enzyme activities related 

 to the needs for synthesis of phage DNA. The most remarkable instance is 

 the appearance of an enzyme that catalyzes the hydroxymethylation of de- 

 oxycy tidy lie acid (Flaks and Cohen, 1957) in bacteria infected with the 



