IV RNA AND GROWTH PROTEIN SYNTHESIS 273 



the view that RNA plays an essential part in protein synthesis — obviously an es- 

 sential process in growth. This evidence has been recently reviewed in detail by 

 Brachet (1955a); therefore, only the main points need to be examined here. 

 We shall successively deal with the cytochemical and quantitative facts in favour 

 of the hypothesis, the indirect evidence based mainly on the study of plant viruses 

 and finally the more direct experiments of the past two or three years. 



(a) Cytochemical and quantitative evidence 



The hypothesis that RNA plays an essential part in protein synthesis has been 

 proposed simultaneously and independently by Caspersson (1941) and Brachet 

 (1941a) in order to explain their cytochemical findings. Using two different 

 methods (U.V. microspectroscopy, and staining with basic dyes combined with 

 ribonuclease digestion), both workers came to the conclusion that only the cells 

 which are synthesizing large amounts of proteins are rich in RNA. For instance, 

 the exocrine part of the pancreas, the pepsin-producing cells in the gastric mucosa, 

 liver cells, nerve cells, growing oocytes and embryos, and silk glands in silk worms, 

 all have a very high RNA content. On the contrary, certain tissues (such as kidney, 

 heart and muscle) possess a very high physiological activity without synthesizing 

 proteins; these are always poor in RNA. The tissues which show marked mitotic 

 activity, like spleen, lymph glands or testis, have an intermediary RNA content. 



A considerable number of investigations have confirmed these early findings 

 and we can only refer to a few of the more recent papers dealing with the subject: 

 in pituitary, for instance, stimulation of hormone synthesis is linked with a marked 

 increase in RNA content (Desclin, 1940; Herlant, 1943; Abolins, 1952). The 

 same relationship holds in the salivary glands of Drosop hi I a larvae (Lesher, 1951) 

 and in seminal glands of rats, where protein synthesis can be stimulated with 

 testosterone (Rabinovitch et al., 195 1). According to Schrader and Leuchtenber- 

 ger (1950), protein and RNA synthesis are closely linked together during sper- 

 matogenesis, while DNA synthesis is independent of protein increase. Inhibition 

 of growth and protein synthesis in the unicellular alga Acetabularia mediterranea by 

 suppression of light (Stich, 1951) or metabolic poisons (Brachet, 1952a) leads to a 

 marked decrease of the RNA content of the nucleolus. 



Still more recently it has been shown by Ficq and Brachet ( 1 956) that the in- 

 corporation of a labelled amino acid (phenylalanine) into the various tissues of 

 the mouse runs exactly parallel with cytoplasmic basophilia, i.e. RNA content: 

 incorporation is highest in the pancreas (except in the Langerhans islets, which are 

 8 times less active than the exocrine part) , and very active in the basophilic parts of 

 the intestinal mucosa, liver, spleen and uterus. There is very little incorporation in 

 tissues which are poor in RNA, like muscle, heart and lung for instance (Fig. 4a, b). 



These few examples should be suflficient to show that the relationship existing 

 between RNA and protein synthesis is a very general phenomenon, occurring in 

 all living organisms, and under very different physiological conditions. 



The cytochemical observations which have just been briefly summarized have 

 been confirmed by a large number of quantitative biochemical estimations of the 

 RNA content of various tissues. As shown by Brachet (1941b) and by Davidson 

 (1947), there is no doubt that the protein-synthesis activity of a tissue is altered 



Literature p. sgg 



