274 KLLIOT VOLKIN 



force the conclusions of XOlkiii and Astracliaii hut additionally describe 

 the subcellular localization of the new RNA, and, in one instance, dem- 

 onstrate a bona fide coniplcxiiiij; bet\v(>en iiha<j;e-s])e('iric RNA and tlie 

 homologous phage DNA. 



By employing a sucrose gradient technifjue for the fractionation of 

 bacterial intracellular particles, Nomura et al. (1960) showed that the 

 SOS particles contained most of the radioactive RNA after a short-term 

 lalnding period of T2-iiif('ctcd E. coli with P'^-. A large pai't of this 

 radioactive RNA could be released, depending on the Mg'" concentration, 

 yielding a nucleic acid essentially free of pi-otein with a sedimentation 

 constant of 8S. Some of the latter exists in the "soluble" foi-m in the 

 T2-infected cell. The T2-s]iecific RNA has a distinguishably higher elec- 

 trophoretic mobility than normal ribosomal RNA. 



Brenner et al. (1961) employed the elegant procedure of separating 

 components of differing densities by equilil)rium centrifugation in a 

 cesium chloride gradient. They, too, concluded that RNA with a base 

 composition similar to that of phage DNA is associated with ribosomes 

 and can be detached by lowering the magnesium concentration. How- 

 ever, in this case, the new RNA is associated with 70S and lOOS ribo- 

 somes, and when disassociated has a sedimentation constant of 12S. In 

 accord with the findings of Volkin and Astrachan, their data clearly 

 demonstrate a turnover of the newly synthesized RNA, whether labeling 

 was carried out with C"-uracil or P^-04. They further demonstrated that 

 no new ribosomes are formed after T2 infection, but that these particles 

 become enriched for T2-specific or T2 messenger RNA. Similar experi- 

 ments employing S^^ for protein labeling revealed that the ribosomes are 

 also enriched for nascent protein that undergoes rapid turnover with 

 respect to its association to the ribosomes. These data led them to the 

 conclusion that pre-existing ribosomes synthesize most of the protein in 

 the infected cells but under the complete direction of the messenger RNA 

 they contain, uninfluenced by the bulk of stable ribosomal RNA. 

 Gros et al. (1961), using sucrose gradient separation techniques, con- 

 cluded that normal, actively growing E. coli contains a messenger RNA 

 component, physically distinct from the bulk ribosomal RNA and trans- 

 fer RNA. They find the active RNA associated with 70S ribosomes, 

 which Tissieres et al. (1960) have described as the principal site of 

 in vitro protein synthesis. Turnover of the messenger RNA in uninfected 

 E. coli was clearly demonstrated ; in this case, however, turnover consists 

 of a transfer of newly labeled RNA to metabolically stable ribosomal 

 RNA and soluble RNA, rather than a breakdown of RNA to acid-soluble 

 products. 



