(a) GROUP I 

 20 HOUR CHASE 



3. 



E 



o 

 u> 



CVJ 



O 



(b) GROUP 2 



CONTROL 

 3 OAY CHASE 



10 20 



TUBE NUMBER 



Fig. 4. 



30 



6000 

 4500 

 3000 

 1500 

 



o 



T) 



6000 

 4500 ? 

 -^3000 

 1500 







6000 

 4500 

 3000 



-1500 



Sedimentation patterns of RNA isolated from "CO2- 

 labeled control and Immobilized magnesium starved 

 embryos: a) 35 Group 1 control embryos, b) 35 Group 

 2 embryos, 3 day chase, c) 35 Group 3 magnesium de- 

 ficient embryos. The embryos were incubated for 1 hour 



in a solution containing 10 fic/ml Naj 

 embryos were used in this experiment. 



14 



CO,. Sibling 



magnesium deficient embryos had a specific 

 activity of 11,700 CPM/OD (Fig. 4c), Since 

 the specific activity of the R-RNA of the group 

 2 embryos was lower than specific activity of 

 the R-RNA from magnesium deficient embryos 

 the group 2 embryos were making more ribo- 

 somes than the magnesium starved embryos. 

 The question still remained whether the 

 magnesium starved embryos converted all newly 



synthesized R-RNA into ribosomes. When ac- 

 tinomysin D was used to inhibit RNA synthesis 

 in HeLa cells, the majority of the newly syn- 

 thesized R-RNA remained in the nucleus in the 

 form of 28S and 18S R-RNA subunits and was 

 not assembled into ribosomes. Some of the 

 R-RNA was assembled into ribosomes which 

 were transferred to the cytoplasm (5). An ex- 

 periment was performed to test the possibility 

 that a similar situation existed in magnesium 

 deficient embryos. 



Control and immobilized magnesium defi- 

 cient embryos were labeled with ''^COj for 1 

 hour and then placed in non- radioactive medium 

 for a 20-hour "chase". RNA was extracted from 

 control and magnesium starved embryos and 

 ribosomes isolated from control and magnesium 

 starved embryos. The RNA was then analyzed in 

 a sucrose density gradient. The specific activi- 

 ties of the 28S R-RNA peaks were determined 

 for each sample. If the ratio of the specific 

 activities of whole egg 28S RNA/28S RNA ex- 

 tracted from isolated ribosomes was appre- 

 ciably higher for magnesium starved embryos 

 than the same ratio for control embryos, it 

 would indicate that the magnesium starved em- 

 bryos have difficulty in assembling newly made 

 R-RNA into cytoplasmic ribosomes. 



The value of the ratio was 0.98 for mag- 

 nesium starved embryos and 0.97 for control 

 embryos. These figures indicated that no more 

 newly synthesized R-RNA was accumulated in the 

 nuclei of magnesium starved embryos than was 

 accumulated in the nuclei of control embryos. 



The data presented here indicate that im- 

 mobilized magnesium deficient embryos contain 

 almost normal amounts of ribosomes and are 

 capable of synthesizing ribosomes. These mag- 

 nesium starved embryos, however, made fewer 

 ribosomes than did control embryos of the same 

 chronological age. The experiments on magne- 

 sium deficient embryos in this report were 

 based on the assumption that ribosome synthesis 

 in the magnesium deficient embryos was, some- 

 how, impaired. It must be remembered, how- 

 ever, that a characteristic of the magnesium 

 starvation syndrome is partial developmental 

 arrest of the magnesium deficient embryos. 

 Control and magnesium starved embryos of the 

 same chronological age were not at the same 

 developmental stage. It is possible that the mag- 

 nesium deficient condition had no effect at all 

 on the rate of synthesis of ribosomes and that 

 the rate of ribosome synthesis observed in the 

 magnesium deficient embryos was characteris- 

 tic of all embryos at that developmental stage. 



40 



