358 MAHLOX B. HOAGLAND 



dergo, as a function of the Mg ++ concentration of the medium. Thus, in 

 general, when one lowers the Mg ++ concentration from 0.005 M to below 

 0.001 M, the 80 S particles in large measure disappear and particles of 

 roughly 60 and 40 S appear. The molecular weight of the former is twice 

 that of the latter and the sum of the molecular weights is that of the orig- 

 inal 80 *S particle. Further dissociation of ribosomes occurs if the Mg++ 

 concentration is reduced below 0.001 M , some of which may be reversible. 

 Clearly irreversible breakdown supervenes if the Mg++ concentration falls 

 below 0.0001 M with the appearance of a variety of sedimentation peaks 

 of *S values below 20. In the presence of excessive Mg ++ , on the other hand, 

 irreversible degradation 44 and precipitation of aggregates (Petermann et 

 aL 16b ) have been reported to occur. 



(3) Chemistry of Nucleic Acid and Protein Components. It has been re- 

 ported by Ts'o 46 for plant particles and Petermann 49 for mammalian par- 

 ticles, that the divalent ion content of the intact particles is very high — 

 approximately 4 moles of ion per mole of base, with Mg ++ in excess of Ca ++ 

 by a factor of 6 or 7. 46 There is some indication 46 that these ions are bound 

 chiefly to the RNA component, rather than to the protein. 



The purest ribosomes appear to be formed almost wholly of RNA and 

 protein. Although there are differences in the ratio of one to the other, the 

 RXA content is high — from 40 to 65 %. The molecular weight data suggest, 

 as pointed out by Tissieres and Watson, 44 that of the two components the 

 RXA may be the invariant of the two, the protein being in greater or lesser 

 amounts in different tissues. It is of interest to note that the subunits ob- 

 tained by dissociation in low Mg ++ medium have the same RNA to protein 

 ratio as the parent complex. 



It is of basic importance to enquire: what is the minimal subunit of RNA 

 naturally occurring in ribosomes? Studies of this problem are in their very 

 early stages. The work is hampered by the natural occurrence of ribonu- 

 clease in close association with most of these particles which may give 

 spuriously low molecular weight values. Preliminary studies indicate, how- 

 ever, that subunits of particulate RNA of molecular weight as low as 250,000 

 to 120,000 may occur 46 (Hall and Doty 16h ). A minimal value of 500,000 

 would perhaps be a more cautious estimate. These RNA subunits may 

 represent physiological entities bound to each other or to the particle pro- 

 tein by Mg ++ and base interaction. However, much more study will be 

 required before this matter is fully clarified. Ts'o and Squires, 50 for example, 

 have isolated RNA from pea seedling and reticulocyte particles in 90 % yield 

 using 1 % dodecyl sulfate. Ultracentrifugal analysis of this material showed 

 that about half of the material sedimented with S 2 o,w of 28 S and half with 



49 M. L. Petermann and N. A. Mizen, Abstr. Meeting Biophys. Soc. p. 1 (1959). 



50 P. O. P. Ts'o and R. Squires, Federation Proc. 18, 1351 (1959). 



