RNA IN THE SYNTHESIS OF PROTEINS 



characterized soluble and ribosomal RNA's. Immediately they observed that 

 none of the T2 RNA was incorporated into stable ribosomes. Instead, in low 

 Mg ++ ( io -4 M) it existed free while in io~ 2 MMg++ they thought it became 

 part of 30s ribosomal like particles. At the same time, Mr. Risebrough in our 

 laboratories began studying T2 RNA, also using sucrose gradient centrifuga- 

 tion. He also found that T2 RNA was not typical ribosomal RNA. In addi- 

 tion, he was the first to notice (in early spring i960) that in io~ 2 M Mg ++ , 

 most T2 RNA scdimented not with 30s particles but with the larger 70s and 

 ioos ribosomes. 



His result leads naturally to the hypothesis that phage protein synthesis 

 takes place on genetically non-specific ribosomes to which are attached 

 metabolically unstable template RNA molecules. Independently of our work, 

 Brenner and Jacob motivated by the above-mentioned metabolic and genetic 

 experiments from the Institut Pasteur, were equally convinced that condi- 

 tions were ripe for the direct demonstration of metabolically unstable RNA 

 templates to which Jacob and Monod 36 gave the name messenger RNA. In 

 June of i960, they travelled to Pasadena for a crucial experiment in Mesel- 

 son's laboratory. They argued that all the T2 messenger RNA should be 

 attached to old ribosomes synthesized before infection. This they elegantly 

 demonstrated 37 by T2 infecting heavy ( I3 C and I5 N) labeled bacteria in light 

 ( I2 C and I4 N) medium. Subsequent CsCl equilibrium centrifugation revealed 

 that most of the T2 messenger RNA was indeed attached to « old » ribosomes, 

 as was all the ribosomal bound nascent protein, labeled by pulse exposure to 

 radioactive amino acids. 



Demonstration of Messenger RNA Molecules in Uninfected Bacteria 



We were equally convinced that similar messenger RNA would be found in 

 uninfected bacteria. Its demonstration then presented greater problems, be- 

 cause of the simultaneous synthesis of ribosomal and soluble RNA. Francois 

 Gros had then (May i960) just arrived for a visit to our laboratory. Together 

 with Mr. Kurland and Dr. Gilbert, we decided to look for labeled messenger 

 molecules in cells briefly exposed to a radioactive RNA precursor. Exper- 

 iments with T2 infected cells suggested that the T2 messenger comprised 

 about 2-4% of the total RNA and that most of its molecules had lives less 

 than several minutes. If a similar situation, held for uninfected cells, then 

 during any short interval, most RNA synthesis would be messenger. There 



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