428 F. GROS 



protein and are probably associated with particles or granules of high in- 

 trinsic metabolic activity. 



d. Incorporation of Radioactive Precursors into the Nucleic Acids 



Although this chapter is concerned with protein formation, the integra- 

 tion of nucleic acid and protein synthesis is of such an importance that 

 some problems dealing with the mechanism of RNA formation deserve 

 particular discussion. These problems relate to the role of the "free" amino 

 acids in the synthesis of RXA, and to the possible nature of the interme- 

 diates in the formation of the RNA particles. The first problem will be con- 

 sidered later (see Section IV). In a consideration of the mechanism of for- 

 mation of the particulate RXA three possible precursors must be examined: 

 (1) the sRNA itself, (2) the RNA fraction associated with the membranes, 

 and (3) the free RNA of high molecular weight. 



e. sRNA as a Precursor of the Stable RNA? 



When adenine-C 14 is added to an exponentially growing culture of E. 

 coli, radioactivity appears much earlier in thesRNA than in the "RNA par- 

 ticles (pRNA)" (Fig. 4). 122 Similar observations have been made using P 32 . 123 



If to the bacteria labeled in the steady state, an excess of adenine-C 12 is 

 added, the specific radioactivity in the sRNA is appreciably diminished 

 while it rises in the RNA of the particles. 



Several mechanisms could account for these observations: 



(1) Fixation of nucleotides to the terminal position of the sRNA (BTP = 

 nucleoside triphosphate; BMP = nucleoside monophosphate), according 

 to the reaction first described by Hecht el al. 109 which prepares the sRNA 

 for its combination with activated amino acids 



BTP + sRNA ^ BMP-sRNA + PP 



BMP-sRNA + ENZ-adenyl-AA -> AA-BMP-sRNA + AMP 



(2) Conversion of the sRNA to the stable "RNA particles" 



n sRNA -» pRNA 



or 



n sRNA — > n' nucleotides — > pRNA 



(3) Fixation of nucleotides onto the end group of the sRNA, followed by 

 their incorporation into the "RNA particle" 



n sRNA + n BTP -► n BMP-sRNA + n PP 



n BMP - sRNA -» pRNA + sRNA 



122 S. Naono and F. Gros, unpublished data (1959). 



123 S. Naono, unpublished (1958). 



