RNA IN THE SYNTHESIS OF PROTEINS 



collection of molecules of different base sequences, synthesized on the func- 

 tioning regions of chromosomal DNA. Following their synthesis, they 

 combined with the basic ribosomal proteins to form ribosomes. We thus 

 visualized that the seemingly morphological identical ribosomes were, in 

 fact, a collection of a very large number of genetically distinct particles 

 masked by the similarity of their protein component. 



Then there existed much suggestive evidence that ribosomal RNA mol- 

 ecules were stable in growing bacteria. As early as 1949, experiments showed 

 that RNA precursors, once incorporated into RNA, remained in RNA. Then 

 the distinction between ribosomal and soluble RNA was not known, but 

 later experiments by the ribosomc group of the Carnegie Institute of 

 Washington and at Harvard indicated similar stabilities of both fractions. 

 These experiments, however, did not follow the fate of single molecules, and 

 the possibility remained that a special trick allowed ribosomal RNA chains 

 to be broken down to fragments that were preferentially re-used to make 

 new ribosomal RNA molecules. Davern and Mcselson 30 , however, ruled out 

 this possibility by growing ribosomal RNA in heavy ( I3 C, I5 N) medium, 

 followed by several generations of growth in light ( I2 C, I4 N) medium. They 

 then separated light from heavy ribosomal RNA in cesium formate density 

 gradients and showed that the heavy molecules remained completely intact 

 for at least two generations. This result predicts, assuming ribosomes to be 

 genetically specific, that the protein templates should persist indefinitely in 

 growing bacteria. 



Experiments Suggesting Unstable Protein Templates 



But already by the time of the Davern 6V Meselson experiment (i959)> 

 evidence began to accumulate, chiefly at the Institut Pasteur, that some, if 

 not all, bacterial templates were unstable with lives only several per cent of a 

 generation time. None of these experiments, by themselves, were con- 

 vincing. Each could be interpreted in other ways which retained the concept 

 of stable templates. But taken together, they argued a strong case. 



These experiments were of several types. One studied the effect of sud- 

 denly adding or destroying specific DNA molecules. Sudden introduction 

 was achieved by having a male donor introduce a specific chromosomal 

 region absent in the recipient female. Simultaneously the ability of the male 

 gene to function (produce an cnzymatically active protein) in the female cell 



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