248 PHILIP SIEKEVITZ 



sub-units have the same RN A /protein ratios, indicating that the Mg^ + is 

 invohed in RNA to RNA binding and not in RNA to protein binding. 

 The reasons for this apparent discrepancy between these resuhs and ours 

 is not known (cf. [7]); it could be that while in the pea seedling particles 

 there is about i mole Mg + + for every four phosphates in the RNA [13] 

 and while the Mg + + requirement for the E. coli particles seems to be very 

 high [11], in the pancreatic particles there is only about i Mg + + for every 

 ten phosphates [7]. 



In our view, there are three components to be considered in the 

 pancreatic RNP particles ; the RNA and proteins themselves, and the sub- 

 divisions of the latter into the so-called "structural" proteins of the 

 particles, and into the proteins which are being synthesized by the particle 

 and some of which still reside there, on the "template" if you wish. The 

 reason for the latter part of the statement are the earlier findings [7] and 

 the present ones, that Mg + + chelators and replacers can release some of 

 the proteins of the particle without releasing the bulk of them. Even the 

 RNA of the particle might not be homogeneous, for we have never been 

 able to remove all the RNA from the protein of the particle, either by Mg + + 

 chelators [7] or by the removal of the spermine in the present experiments. 

 Thus it could be that a small proportion of the RNA is more tightly bound 

 to the protein, and could be metabolically different, than is the bulk of 

 the RNA. 



What are the probable bindings between RNA and proteins in the 

 pancreatic particles? There is evidence that hydrogen bonding [15, 16], 

 electrostatic bonding [17] and Mg + + complexing (cf. literature cited in 

 [7]) are all somehow involved. I will just discuss the Mg + + binding and the 

 effects of spermine on this binding. We must look to two possibilities ; that 

 the Mg + + ties together various RNA chains in the RNP and thus stabilizes 

 the hydrogen and electrostatic bonds between the RNA and the protein 

 of the RNP [15], or that Mg + + links the RNA chains both to the structural 

 and to the newly synthesized proteins of the RNP (cf. [7]). It is difficult 

 to decide between these two not mutually exclusive propositions, for it is 

 conceivable that if Mg + + complexes adjacent RNA chains, the removal of 

 this Mg + + so weakens the hydrogen and electrostatic bonds between 

 RNA and protein that these groups become separated from each other. 

 It is thus clear that no one binding force seems adequate by itself to explain 

 the structural stability of the particles. However, it would appear that the 

 Mg + + complexing is of utmost importance, at least in the pancreatic 

 particles. Once the Mg + + is removed, and not replaced by other binding 

 agents, such as spermine, then any hydrogen bonds or ionic forces which 

 might hold the RNA to the protein are weakened, and the particle loses 

 identity as an RNP particle. In the case of the particles from other sources, 

 it would appear as mentioned, that Mg + + links together the RNA chains 



