J 0.75 



I 0.6 



SO.45 



^ 0.3 

 O 



0.15 

 



(a) CONTROL EMBRYOS 



10 



(b)Mg. STARVED EMBRYOS 



20 10 



TUBE NUMBER 



loii 



Fig. 3. 



Sedimentation patterns of RNA extracted from ribosomes isolated from '^C02 -labeled 

 control and immobilized magnesium starved embryos: a) 25 control embryos, b) 25 mag- 

 nesium starved embryos. The embryos were incubated for 1 hour in a solution containing 

 10 //c/ml of Na2'''C03 and then placed in non-radioactive solution. 



of the 20-hour "chase" the embryos were sep- 

 arated into three groups. RNA was extracted 

 from the ribosomes isolated from one group 

 (group 1) of embryos. A second group (group 2) 

 was placed in 10% Holtfreter's solution con- 

 taining magnesium. The third group (group 3) 

 was placed in 10% Holtfreter's solution which 

 lacked magnesium. The second and third groups 

 of embryos were kept in their respective solu- 

 tions for three days, at which time ribosomes 

 were isolated from each group of embryos and 

 extracted for RNA. The magnesium starved 

 embryos were immobile by the end of three 

 days. The RNA obtained from each group of 

 embryos was analyzed by sucrose density gra- 

 dient centrifugation. If the ribosomes of the 

 magnesium deficient embryos were stable, the 

 amount of radioactivity present in the R-RNA 

 of immobilized magnesium starved embryos 

 would be identical to the amount of radioactivity 

 present in the R-RNA of an equal number of 

 embryos from each control group (Group 1 and 

 group 2). 



The amount of radioactivity present in the 

 R-RNA of immobile magnesium deficient em- 

 bryos was equal to the amount of radioactivity 

 present in the R-RNA of group 2 embryos and 



very nearly equal to the amount of radioactivity 

 present in R-RNA of group 1 embryos (Figs. 

 4a, b, c). This demonstrated that the magnesium 

 starvation syndrome did not affect the stability 

 of normal ribosomes. 



This same experiment also indicated that 

 the synthesis of ribosomes was slower in mag- 

 nesium starved embryos as compared to con- 

 trol embryos. The specific activities, measured 

 in counts/minute/unit of optical density at 260 

 mu (CPM/OD), of R-RNA from group 1, 2 and 

 3 embryos were presumably identical at the 

 end of the 20-hour chase. Since no more radio- 

 activity was available for R-RNA synthesis in 

 group 2 and 3 embryos (the total radioactivity 

 incorporated into the RNA was nearly the same 

 for each group), any further synthesis of R-RNA 

 would result in a dilution of the radioactivity 

 and a reduction in the specific activity of the 

 R-RNA. The specific activities reported here 

 were calculated from the amounts of radio- 

 activity and optical density present in the peak 

 tube of the 28S R-RNA component of each of the 

 three groups. The specific activity of the R-RNA 

 of group 1 embryos was 16,700 CPM/OD (Fig. 

 4a). Group 2 R-RNA had a specific activity of 

 9700 CPM/OD (Fig. 4b), while the R-RNA from 



39 



