TS'O 165 



the sedimentation coefficients of the 60 and 80 S particles of E. coli reported by 

 Bolton et al. in this meeting is most probably due to differences in molecular 

 weight. These would then form the series 3:2:1. In addition, particles of 25 S 

 have been found as dissociation products of the 40 S particle of E. coli by 

 Tissieres and Watson, this too having a spherical shape in the electron micro- 

 scope, as observed by Hall. A 25 S component has also been reported by Chao 

 [4] as one of the dissociation products of yeast particles. 



The following scheme is proposed to account for the experimental findings 

 concerned with the dissociation of the 80 S particles. When a certain fraction of 

 the magnesium ion is removed from the system (about 50 per cent as based on 

 the binding constants of RNA and phosphate [5]), the 80 S particle dissociates 

 reversibly to form a 40 S unit, one-third of the original particle, and a 60 S unit, 

 two-thirds of the original particle. When larger amounts of magnesium ions 

 are removed (over 95 per cent, by versene), not only does dissociation of the 

 80 S particles to 40 and 60 S units occur, but in addition the 60 S unit also is 

 degraded, possibly irreversibly, to form a 40 S unit and two 25 S units. The 

 final result of such a dissociation should be a system containing two 40 S units 

 and two 25 S units per original 80 S particle. This formulation, then, fits the 

 experimental findings both as to the mass ratio of 2:1 between 40 and 60 S units 

 and as to the mass ratio of 2:1 between 40 and 25 S components. 



The magnesium and calcium contents of the particle have been analyzed by 

 flame spectrophotometry." There are 3.0 to 3.2 umoles of magnesium per 12 

 umoles of RNA-phosphorus or per 10 mg dry weight of particles. All the mag- 

 nesium appears to be extractable by 0.5 N TCA at 3 C. Assuming that cal- 

 cium can also be completely extracted by TCA, then, again, per 12 umoles of 

 RNA-phosphorus, there is 0.45 to 0.55 umole of calcium, about one-sixth of 

 the amount of magnesium. Thus, the particles contain 3.5 to 3.7 umoles of 

 magnesium and calcium per 12 umoles of phosphate, or 1 mole of divalent ions 

 for each 3.3 ±0.2 umoles of phosphate. 



It is of interest to estimate the apportionment of these divalent ions between 

 the RNA and the protein. Combining the data on molecular weight (4.5 X 10 c ), 

 RNA content (40 per cent), and content of divalent ions per mole of phos- 

 phorus, we may calculate that there are 1.5 XlO 3 moles of magnesium and cal- 

 cium per mole of particle. Since the cation binding capacity of the microsomal 

 protein is unknown, we introduce for comparison the results that may be cal- 

 culated for bovine serum albumin, a protein with a high proportion of dicar- 

 boxylic amino acids. According to Carr [6], there are 8 calcium binding sites 

 per molecule at pW 7.4. The total amount of protein per mole of microsomal 

 particles is equivalent to 36 moles of bovine serum albumin. This protein 

 would therefore bind no more than 300 moles of calcium, and would account 

 for less than 20 per cent of the divalent ions bound by the particles. We con- 



2 The technical assistance of Mr. Merck Robison, Carnation Company, is gratefully 

 acknowledged. 



