Ill PFS OF DIFFERENTIATED AND EMBRYONIC CELLS 5II 



of the microsomal fraction of these organs an RNA phosphorus content of 2.3, i.i, 

 and 0.6% was found, respectively. The corresponding figures for the incorporation 

 of glycine-'^N into cellular proteins after injection into the intact animal were 

 found to be 0.183, 0.107, ^^^ 0.046 atom per cent excess, respectively. This ap- 

 pears to indicate a remarkable parallelism between RNA content and the rate of 

 incorporation of amino acids into the proteins of the microsomal fraction. On the 

 other hand no parallelism was found between the uptake of the microsomal proteins 

 and the uptake of RNA (0.006 and 0.038 atom per cent excess in the RNA of 

 pancreas and liver, respectively). The authors recall results of data presented by 

 Abrams (1951) which show that irradiation of the intestinal epithelium does not 

 impair protein synthesis whereas RNA synthesis is severely inhibited. In contrast to 

 the findings in microbial cells these data indicate that, in these organs, pro- 

 tein formation is related to RNA quantity and not to RNA turnover. As Kihara, 

 et al. (1955) suggest, acceptance of this dual role of RNA already mentioned 

 earlier eliminates the apparent disagreement of the conclusions of Allfrey, et al. 

 (1953) and Hokin and Hokin (1953). 



The exploration of the role of RNA in differentiated cells has been described in 

 detail in order to indicate the large number of experimental approaches which 

 have been used in establishing its significance in protein synthesis. In contrast, 

 there is almost a complete lack of systematic information about the relation of 

 RNA quantity and RNA synthesis to protein formation during proliferation and 

 during elaboration of specific cell products during later phases of diflferentiation 

 in embryonic tissues. Indirectly, a relationship between RNA and protein syn- 

 thesis in early embryonic development is suggested, perhaps, by experiments with 

 purine analogs. The conclusiveness of these experiments is limited since analogs 

 were used which are antagonists of both RNA and DNA. Using the purine analogs 

 8-azoguanine and benzimidazole, Waddington, Feldmann and Perry (1955) 

 studied the efifect upon the development of the chick and the newt Triturus Alpestris. 

 In discussing possible explanations of the abnormal forms of development caused 

 by the analogs, Waddington dismisses interference with cell division since none of 

 the sensitive tissues such as the gastrula or the sensitive mesodermal derivatives 

 (somites) show a particularly high mitotic rate normally. The hypothesis is put 

 forward that the greater sensitivity of certain tissues at certain phases of develop- 

 ment is due to an interference with a relatively higher protein synthesis in these 

 cells. In support of this hypothesis, Waddington points out that methionine--^-^S 

 is incorporated at an apparently increased rate in the primitive streak region of 

 the chick blastoderm (gastrulation) while the invaginated tissues showed much 

 less activity in radioautographs. The same intense incorporation was noted in the 

 neural fold which also proved sensitive during the earlier exposure time to azo- 

 guanine (18-21 h.). At later stages methionine accumulated particularly in the 

 anterior portion of the neural tube and at still later stages again in the somites of the 

 chick embryo, these also being the regions with the most marked purine analog 

 sensitivity. Waddington points out that the effects of purine analogs are not paral- 

 leled in detail by the effects of ethionine. Since ethionine is supposed to inhibit 

 protein synthesis directly, this discrepancy requires further investigation. Objec- 

 tions to an interpretation of radioautographs in terms of rates of protein synthesis 



Literature p. sjg 



