Sedimentation coefficients of RNA from 70S and 80S ribosomes.

It has been shown' previously that the corrected sedimentation coefficients (820 W) of the ribosomes from 25 species of fungi and 25 species of bacteria, respectively, range from 76.5 to 84.5 Svedberg units (S) and from 63.8 to 72.1S, and that the difference between the arithmetic means, 81.3 and 68.4S, of these two frequency distributions is significant. Since the ribosomes from other eucaryotic microorganisms, from plants, and from animals also belong to the 80S class,2 it appears that 70S ribosomes are characteristic of bacteria. Therefore, it becomes of interest to determine whether this uniqueness is also reflected in the properties of ribos6mal RNA (rRNA) which, as suggested by Spirin3 and Kono and Osawa,4 probably determines most of the over-all structure of ribosomes. It is generally accepted that ribosomes from various sources yield only two discrete populations ofRNA molecules provided that precautions are taken to eliminate or inhibit contaminating nucleases.3 5-8 These two RNA classes are usually characterized by their s values.2 The bacterial rRNA's are often referred to as 16 and 23S and those from other sources as 19 and 28S. Exceptions to this generalization appear, however, even when the s values are corrected for concentration effects. This situation can be explained, at least in some instances, by the fact that not all rRNA preparations have been analyzed in the same ionic environment. For example, several workers3 8-11 have shown that there is a strong dependence of sedimentation ofRNA on Mg ion concentration. Spirin3 studied the sedimentation of rRNA's from Escherichia coli, rabbit, and peas under the same conditions (0.01 M EDTA and ionic strength of 0.1) and found th~e following respective pairs of values: 16.7 and 23.OS, 14.5 and 24.5S, and 13.9 and 25.1S. From these results it would appear that the s values for rRNA's from higher organisms are not all greater than those from bacteria. In addition, Spirin emphasized the fact that when the s value for the large component increases, the s value for the small one decreases. A similar observation was made by Littauerl2 for the rRNA obtained from chick, rat, and calf. Using an empirical equation to convert s values into molecular weights, Spirin found that regardless of the origin the sum of the molecular weights of the large and small rRNA's equals 1.7 X 106. Since there is no a priori reason to believe that in solution various rRNA's would not have similar secondary structures,"3 14 Spirin's hypothesis of universality would preclude the possibility that the difference between s values for ribosomes reflects the sum of the s values of their RNA's. In view of our previous work' and the fact that Spirin's investigation was limited to only three organisms, we decided to study, under the conditions used by this author, the sedimentation of rRNA from five fungal and five bacterial species. The results show that the sum of the s values for fungal rRNA pairs is larger than that for bacterial rRNA pairs. Furthermore, a significant difference was found between the s values of the smaller rRNA components from fungi and bacteria as well as between the s values of the larger rRNA components of the two groups of organisms.