TABLE I 



Base Compositions of Ribosomal RNA. a. Base composition of 28S and 18S R-RNA sub- 

 units. The values are expressed as mole per cent of the total RNA. b. The 32 p base 

 composition of the ■'2p_ia(,giej[ 28S RNA isolated from early neurula embryos. Values 

 are expressed as the per cent of the total CPM in the ^^P-labeled 28S RNA. 



Material 



a. Frog Eggs 



Adult 

 Frog Liver 



b . Early Neurula 



In comparing R-RNA synthesis in R ana 

 pipiens haploid embryos and the Rana catesbei- 

 ana x Rana pipiens hybrid embryos it is strik- 

 ing that the haploid embryos exhibit the normal 

 pattern of R-RNA synthesis. The addition of a 

 foreign set of chromosomes to the Rana pipiens 

 haploid set of chromosomes has poisoned the 

 hybrid embryo and rendered it incapable of 

 further development. It is not likely that the 

 crippling of the hybrid's ability to elaborate 

 R-RNA was responsible for the developmental 

 retardation and death of the embryo. Recent 

 studies have shown that the anucleolate embryos 

 of Xenopus laevis develop to the swimming tad- 

 pole stage in the complete absence of R-RNA 

 synthesis (3). 



The relative inability of the hybrids to 

 elaborate R-RNA prompted us to utilize another 

 experimental system. A developmental abnor- 

 mality caused by rearing Rana pipiens embryos 

 in medium lacking magnesium seemed to offer 

 an approach to the problem of the control of 

 R-RNA synthesis during development. Embryos 

 reared in this manner (magnesium deficient 



embryos) develop normally to stage 21-23 

 (swimming tadpole) after which they undergo 

 developmental retardation, become edematous 

 and immobile and die 2-3 days later (4). Brown 

 initially made several potentially interesting 

 observations regarding the synthesis of ribo- 

 somes in these magnesium deficient embryos (4). 

 The magnesium deficient embryos apparently 

 contained one-sixth as much R-RNA in the 

 isolatable ribosome fraction as did control em- 

 bryos even though the magnesium deficient 

 embryos contained the same amount of total 

 RNA per embryo as did control embryos. Since 

 R-RNA usually comprises 80-90% of the total 

 cell RNA, it was of interest to investigate the 

 nature of the RNA from the immobile magnesium 

 deficient embryos. 



Initial studies on the ribosomal content of 

 magnesium deficient embryos demonstrated that 

 an almost normal complement of ribosomes (as 

 compared with control embryos) could be iso- 

 lated if the ribosome extraction technique de- 

 signed for Rana pipiens eggs was used. Further 

 studies in which R-RNA was labeled while the 



37 



