20 MOLECULES, VIRUSES, AND BACTERIA 



chemically unique type of RNA whose base composition mimics that 

 of viral DNA. In addition, this RNA has an active turnover, in which 

 the ribose nucleotides are converted to deoxyribonucleotides of viral 

 DNA (Volkin and Astrachan, 1957). It has been a difficult problem 

 to demonstrate a function for this RNA, but it has now been shown that 

 the early steps of phage multiplication, which exclude DNA synthesis, 

 require pyrimidines and adenine, presumably for RNA (Pardee and 

 Prestidge, 1959; Volkin, 1960 ) . 



It can be seen in Figure 1 that DNA synthesis in infected cells 

 stops for some minutes. DNA synthesis resumes at a markedly stimu- 

 lated rate which in some media (e.g., lactate) appears to compensate 

 for the RNA which no longer accumulates (Cohen, 1947). The new 

 DNA made after infection is entirely viral, containing HMC and lack- 

 ing cytosine ( Hershey et al., 1953; Vidaver and KozloflF, 1957 ) . 



The significance of HMC 



The existence of the new base in viral DNA, and the lack of 

 cytosine found normally in bacterial RNA and DNA, suggested that 

 hydroxymethylation of cytosine to HMC converted the former base to 

 a form unsuitable for normal bacterial synthesis. As we shall see, dCTP 

 essential to the synthesis of bacterial DNA is indeed converted very 

 efficiently in infected cells to dCMP, the precursor of the HMC deoxy- 

 ribonucleotide, dHMP. Virulence and parasitism in this system are 

 thus exhibited at the molecular level by appropriating a normal es- 

 sential nucleotide for the formation of a unique viral metabolite. It 

 should be noted that the host DNA is degraded in infection and the 

 dCMP therein contained is freed and converted to viral pyrimidine 

 ( Kozloff et al, 1951; Weed and Cohen, 1951 ) . 



In the studies on the isolation of HMC derivatives from viral DNA, 

 it was soon observed that nucleotides containing this base were present 

 in a stnictiire which resisted enzymatic release (Cohen, 1953). Several 

 workers (Volkin, 1954; Sinsheimer, 1954; Jesaitis, 1957) then showed 

 that in viral DNA, HMC was glucosylated at the hydroxymethyl group. 

 It appears that the presence of glucose does indeed protect to a con- 

 siderable extent phosphodiester bonds involving dHMP. Thus viral 

 DNA carries a significant molecular mechanism for self-protection 

 when present in the DNase-rich medium in the infected cell. However, 

 this may not be the sole reason for the survival of viral DNA. 



In addition, we can note that although the base compositions of 

 the HMC viruses, T2, T4, and T6, are essentially identical ( Wyatt and 

 Cohen, 1953), the glucose contents of these viruses differ considerably 

 (Jesaitis, 1957). Thus, T6 contains mainly large amounts of diglucosyl 

 derivatives of HMC plus non-glucosylated HMC; T4 contains only the 



