Virus Action and Replication 125 



that when DNA of an intemperate phage gets inside the ceU, the 

 cellular DNA is destroyed and can no longer serve as a source of 

 information. The phage DNA then assumes command of the 

 synthetic machinery in the cell via messenger RNA copied from 

 itself by hydrogen bonding between appropriate base pairs. This 

 results in formation by the cells' own ribosomes of phage-controlled 

 proteins only. 



The next point of interest is: Wliat proteins does the virus 

 instruct the cell to make? The DNA of the T-even phages is 

 chemically peculiar in that it contains the base 5-hydroxymethyl- 

 cytosine (HMC) in place of the normally occurring" cytosine (15), 

 and in that some of the HMC residues hav^e glucose attached to 

 them after incorporation into the DNA chain (16). Thus, if 

 replication of the bacteriophage is to occur, a phage-infected cell 

 must be able to perform enzyme reactions not possible in an 

 uninfected cell. In the laboratories of Dr. S. S. Cohen and Dr. 

 Arthur Kornberg it has been demonstrated that phage-infected 

 cells do indeed acquire a large number of new enzyme activities 

 within minutes after infection (17, 18, 19). These are the first 

 proteins formed by the infected cell in order that phage DNA may 

 be replicated. Later in the course of infection, DNA synthesis and 

 structural phage protein synthesis begin. 



Cell lysis, which occurs after several hundred new phages per 

 cell have been produced, is caused by the action inside the cell 

 of a lysozyme which is also formed under the genetic control of 

 the phage (20). 



Many steps in the infectious cycle of the lytic phages are ob- 

 viously well understood on a molecular level. The intricacies of 

 the second type of response to phage infection, the lysogenic 

 response, have not yet been .so thoroughly elucidated. In a cell 

 which is infected by a temperate phage (one capable of inducing 

 lysogeny), as soon as the DNx^ enters the cell, differences from the 

 course of events in infection with a member of the T-series of 

 phages can be detected. Even if the pathway to the lytic response 

 is followed, the cell continues to synthesize cellular protein and 

 RNA (21), and can even be induced to foim new enzymes through- 

 out the latent period of the virus (22). If the cell is to become 

 lysogenized, then the synthesis of RNA, DNA, and protein stops. 



