6 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



used different methods and precursors. For their extensive inves- 

 tigations on mouse, rat, and rabbit, they used ^^S-labeled amino 

 acids (instead of phenylalanine) and a stripping film autoradiog- 

 raphy method (instead of the track autoradiography method). 

 Nevertheless, they also found that incorporation is highest in 

 protein-producing glands (pancreas, gastric mucosa, reticuloendo- 

 thelial cells, neurons). Next come tissues where mitotic activity is 

 important (Lieberkiihn's glands, stratum germinativum of the skin, 

 spermatogonia, follicle cells of the ovary). In the least active tissues 

 (muscle, connective tissue), incorporation is only one-fifthieth of 

 that in the protein-secreting glands. Niklas and Oehlert (1956) 

 conclude that, without any exception, RNA content and incor- 

 poration of amino acids into proteins show excellent parallelism. 

 It is also worth mentioning that mature spermatozoa, which con- 

 tain no RNA, are unable to incorporate amino acids into their 

 proteins (Martin and Brachet, 1959). 



However, it would not be correct to believe that all RNA is always 

 metabolically active. For instance, in amphibian ovaries, strongly 

 basophilic degenerating oocytes are occasionally found. They are 

 almost inert as regards incorporation of amino acids into proteins 

 (Ficq, 1955). Similarly, the testes of mammals often contain a 

 large number of extracellular basophilic bodies. These probably 

 correspond to the extrusion of cytoplasmic RNA during sper- 

 miogenesis. These "residual bodies" show very little, if any, activity 

 for amino acid incorporation. Thus the mere presence of RNA 

 is not enough to stimulate protein anabolism. The RNA itself 

 must be in a metabolically active form, probably related to its 

 architecture at the molecular level. 



The evidence arising from cytochemistry is thus very striking. 

 However, cytochemical methods do not have the same high degree 

 of accuracy and specificity as straight biochemical techniques and it 

 thus becomes important to find out whether the cytochemical 

 evidence is confirmed by quantitative chemical analyses of the 

 RNA content. 



