250 PHILIP SIEKEVITZ 



by assuming that the particle is somewhat an open structure,* and the 

 compactness of the particle can be changed by the availability of bonding 

 forces, in this case, the amine groups of the spermine and the phosphate 

 groups of the RNA (cf. [20]). These variations in the compactness of the 

 particles can explain the variations noted in Table IV. Firstly, it is probable 

 that at pH 7 all the basic groups of the spermine are charged (cf. [21]). The 

 amount of spermine binding to the particles could be determined by the 

 availability of phosphate groups open to salt linkage. The farther apart 

 are the available phosphate groups, then the greater the number of 

 molecules of spermine which are bound, for only one amino group per 

 spermine can be bound. f When the phosphates are closer together, the 

 less spermine molecules will be bound, because all the available phosphate 

 groups in the vicinity can be linked to the amine groups of the same 

 spermine molecule. It thus might be of significance that the amount of 

 spermine bound per RNA goes up by factors of approximately two and 

 four (Table IV), as if one, two, or all four of the amine groups in spermine 

 are becoming involved. Thus, in the various experiments cited in Table IV, 

 the RNP particles as isolated may be greatly different in their compactness 

 of structure. When this structure is loose, more spermine can be bound 

 (Table IV, Exps. 5, 6, 7). In the presence of Mg + +, the intermolecular 

 distances might become smaller owing to the formation of Mg + +-spermine- 

 phosphate complexes (cf. [19]). Hence the available phosphate groups can 

 be linked to more than one amine groups of the spermine molecule, thus 

 less spermine molecules are bound in the presence of Mg + + (Table IV, 

 Exps. 6, 7). When the structure is tight, less spermine is bound (Table 

 IV, Exps. I, 2, 3). In the presence of Mg + +, again Mg + +-spermine- 

 phosphate complexes can be formed, but in this case these complexes can 

 open up a resilient structure, allowing more spermine to be bound (Table 

 IV, Exps. I, 2, 3). As disclosed in Table III, and has been found previously 

 [22] with E. coli particles, the particles have higher RNA /protein ratios 

 in the presence of both spermine and Mg + + than in the presence of either 

 alone. Also, when the particles are incubated in the presence of both 

 spermine and Mg + +, the spermine becomes bound, and no Mg + + or 

 RNA leaves the particle. Furthermore, nucleotide incorporation into RNA 

 by liver preparations can be stimulated by the addition together of spermine 



* The mean diameter of the particles under the experimental conditions 

 mentioned, as found by G. E. Palade, is as follows: As isolated, 150 A; after 

 incubation in sucrose, 180 A; after incubation in ATP, P-P, or versene, 250- 

 300 A; after incubation in spermine, 400 A; after incubation in spermine plus 

 Mg ^ ■*■, 300 A. In some cases, when the RNA is removed from the particles, a less 

 dense centre appears, giving the particle a doughnut appearance [7]. 



t It has been estimated [20] that the distance between the terminal primary 

 amines in spermine is approximately 16*5 A. 



