66 THE BIOSYNTHESIS OF PROTEINS 



fashion. On the other hand the isolation of infectious RNA from several 

 viruses is a fundamental progress not only for the understanding of the 

 process of virus multiplication, but also for the development of our know- 

 ledge of RNA in general. With these RNAs which are endowed with 

 specific biological activities, biochemists will learn how to handle RNA 

 without destroying the structural features which are essential to their 

 biological activities. 



It is very important to keep in mind that so far all the studies on RNA 

 (except a few recent works on virus RNA) have been performed on mix- 

 tures of various RNA fractions, which were most of the time partly 

 degraded. This has been an inexhaustible source of confusion. Innumerable 

 works on nucleic acid metabolism are extremely difficult to interpret for 

 the results always concern a mixture of nucleic acids. When tracers are 

 used, new sources of hidden difficulties are involved. Beside the familiar 

 complications due to dilution of labelled precursors, existence of several 

 pools of precursors, interconnexions between metabolic pathways, a very 

 serious type of complication has been appreciated only recently (Vincent 

 and Baltus, 1959). It was found that certain RNA fractions readily bind 

 two molecules of cytidylic acid and one of adenylic acid at the end of their 

 molecule (Heidelberger et al., 1956; Canellakis, 1957; Edmonds and 

 Abrams, 1957; Paterson and Lepage, 1957; Hecht et al., 1958; Herbert, 

 1958; Allfrey and Mirsky, 1959; Harbers and Heidelberger, 1959; Okazaki 

 and Okazaki, 1959). This process was often taken as reflecting nucleic acid 

 synthesis ; it now appears that it corresponds to a metabolic activity which 

 might be connected with protein synthesis. 



All this serves to emphasize the physiological heterogeneity of nucleic 

 acids, the complexities of their metabolism, and the primitive stage in 

 which we are still in our attempts to analyse the function of these substances 

 in biochemical terms. It is important to keep this in mind and to avoid 

 being too dogmatic in all interpretations of RNA metabolism in terms of 

 RNA formation. 



2. Metabolism of RNA and Protein Synthesis 



One of the first working hypotheses retained by workers interested in the 

 function of RNA in protein synthesis, was that RNA and protein must be 

 made simultaneously or that the synthesis of protein depends on RNA 

 formation. A very great amount of research has been devoted to testing 

 these ideas, and it would be hopeless to try and analyse the results of all 

 these contributions. It has repeatedly been reported, for instance, that in 

 growing tissues or in growing bacteria, protein and RNA synthesis go 

 together, or that the rate of incorporation of precursors into the two types 

 of macromolecules depends on the same basic conditions, like the provision 

 of energy, a carbon source, a nitrogen source and active phosphorylations. 



