VI. RNA AND CODING PROBLEMS 273 



II. Evidence Implicating an RNA Species as a 

 Carrier of Genetic Information 



a. the dna-like rna in bacteriophage infection 



In 1956 Volkin and Astrachan observed that in bacteriophage- 

 infected Escherichia coli B a minor species of RNA was synthesized 

 whose composition appeared to be similar to the DNA of the phage but 

 not to the host. This biologic system offers a unique advantage for such 

 a study, since viral infection immediately eliminates the bulk of RNA 

 synthesis, and the synthesis of a minor class of RNA having a different 

 over-all composition can be magnified. Thus, it was possible to demon- 

 strate the existence of phage-specific RNA even during T7 infection 

 (Volkin et al., 1958), as well as that produced during T2 infection. The 

 DNA composition of T7 deviates only slightly from that of the host, 

 whereas the DNA composition of T2 is widely different. The demonstra- 

 tion of phage-specific RNA originally took the form of a comparison 

 between relative specific activities of the RNA mononucleotides liberated 

 by alkaline hydrolysis, after an assimilation of P'^- orthophosphate 

 (Volkin and Astrachan, 1956a,b, 1957; Volkin et al., 1958). The conclu- 

 sions from these experiments, that a phage DNA-like RNA was being 

 synthesized, has since been amply confirmed by employing a C^*-labeled 

 precursor (Volkin, 1959), and by a comparative study of the rates of 

 labeling of the free nucleotide pool with the rates of labeling of the RNA 

 mononucleotides (Astrachan, 1960; Volkin, 1962). 



By employing only brief isotope assimilation periods (pulse experi- 

 ments), it was revealed that the newly synthesized RNA does not 

 merely accumulate but undergoes constant breakdown and renewal, or 

 turnover (Volkin and Astrachan, 1957; Astrachan and Volkin, 1958). 

 It was demonstrated (Astrachan and Volkin, 1958) that such turnover, 

 though always present to some extent during the intracellular production 

 of phage, is most pronounced during the first 10-15 minutes after the 

 initial infection. It is noteworthy that virtually all of the extensive 

 biochemical transformations of the host take place during this initial 

 time period (Romberg et al, 1959; Flaks et al., 1959; Bessman, 1959; 

 Sommerville and Greenberg, 1959; Keck et al., 1960). 



B. GRADIENT CENTRIFUGATION OF CELL COMPONENTS 



The question of a specific RNA newly induced by bacteriophage 

 infection has been reinvestigated in a number of other laboratories by 

 different experimental approaches. Their investigations not only rein- 



