BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 465 



Biochemical studies indicate that ho§t cell metabolism after infection is 

 almost exclusively diverted to phage production (Chapter 26). Following 

 infection, total protein synthesis goes on, and after a few minutes delay 

 there is active DNA synthesis, but normal host enzymes or host DNA are 

 apparently no longer being formed. The respiration and metabolism, which 

 but for the infection would have been increasing in rate, remain more or 

 less constant while the phage multiplies several hundredfold. This in itself 

 suggests that the enzymes whose action is being observed are those of the 

 no longer growing host cell, and not enzymes attributable to the phage. 

 Not only do the host's respiratory and adaptive enzymes fail to increase 

 after infection, but the cell PNA does not increase or undergo metabolic 

 exchange. It has been suggested that intermediates for PNA synthesis are 

 shunted toward DNA synthesis,'*^ but there is little direct evidence to 

 support this view; furthermore, certain enzymes which may be involved 

 in ribose phosphate formation are still active after the infection.^** 



Clearly defined shifts in the synthesis of pyrimidine bases, however, occur 

 under the influence of phage DNA. The already mentioned 5-hydroxy- 

 methylcytosine, never observed in the uninfected host, begins to be syn- 

 thesized soon after infection with the T-even phages, which contain this 

 base. ^^'"2 Also, a strain of E. coli which requires preformed thymine for 

 growth begins rapidly to produce this base when infected with phage. ^®^ 

 These may be looked upon as evidences that phage DNA exerts a profound 

 organizing effect, as well as what migl;it be merely adaptive shifts, in the 

 metabolism of the bacterial cell which it infects. 



Certain phage properties have been observed in mutant forms and have 

 been followed through genetic processes. These are chiefly properties relat- 

 ing to the adsorbability on different bacterial strains or to the mechanism of 

 lysis and consequently to plaque formation) . Except that they may further 

 mutate, these traits are reproduced indefinitely in a pure phage line, pre- 

 sumably being carried on through the specific DNA. New combinations, 

 however, are produced when suitable dissimilar, but related, phages simul- 

 taneously infect the same cell.'*^"'*^ Since these new combinations of prop- 



1" L. M. Kozloff, K. Knowlton, F. W. Putnam, and E. A. Evans, Jr., /. Biol. Chem. 



188. 101 (1951). 

 1" A. L. Koch, F. W. Putnam, and E. A. Evans, Jr., J. Biol. Chem. 197, 105 (1952). 

 "9 F. W. Putnam, D." Miller, L. Palm, and E. A. Evans, Jr., /. Biol. Chem. 199, 177 



(1952). 



180 L. W. Labaw, J. Bacteriol. 62, 169 (1951). 



181 G. S. Stent and O. Maal0e, Biochim. et Biophys. Acta 10, 55 (1953). 



182 O. Maal0e and N. Symonds, /. Bacteriol. 65, 177 (1953). 



183 S. S. Cohen, Nature 168, 746 (1951). 



18^ S. S. Cohen and L. Roth, J. Bacteriol. 65, 490 (1953). 



185 M. Delbrtick and W. T. Bailey, Jr., Cold Spring Harbor Symposia Quant. Biol. 11, 

 (1946). 



