WAGMAN AND TRAWICK 15 



The very slight concentration dependence of S20, w (with a value, at infinite 

 dilution, of 46.9 S) is in agreement with the finding by Schachman et al. [2] that 

 these particles appear to be roughly spherical in the electron microscope. On 

 the assumption of a negligible concentration dependence for diffusion, the 

 molecular weight calculated from the measured data is 1,240,000, and the fric- 

 tional ratio, f/fo, is 1.16, corresponding to spheres, 137 A in diameter, hydrated 

 to the extent of 0.37 ml/g of RNP. 



An electron micrograph, made using a water solution of RNP, is shown in 

 figure 5. The particles appear to be uniformly spherical with a mean diameter 

 about 152 A. This value is in good agreement with the sedimentation-diffusion 

 figure, inasmuch as the probable errors with the electron microscope tend to 

 yield too high a figure. 



DISCUSSION 



It has been reported (e.g., by Chao and Schachman [8] and by Bolton, Hoyer, 

 and Ritter, paper 3 of this volume) that more rapidly sedimenting components 

 (60 S and 80 S) appear in extracts from microbial cells when magnesium ions 

 are present in sufficient concentration in the extract media. This addition to 

 the ionic environment is not required for the preservation of the 47 S particles 

 dealt with in the present report. In the intact cell, the synthesis and dissocia- 

 tion of RNP particles are probably controlled by several factors which may 

 include magnesium and other ions. The action of ribonuclease, for example, 

 is inhibited by a large number of substances [9] including bivalent cations and 

 mononucleotides, all of which are apparently present in the dialyzable fraction 

 of E. coli extracts. 



Although the RNP component has been identified with a variety of enzymatic 

 functions, it is not clear whether these are intrinsic or merely adsorbed. Elec- 

 trophoretic data [6] show that this material, despite its apparent monodispersity 

 in the ultracentrifuge and electron microscope, is heterogeneous, consisting of 

 two or more components with a wide range of anodic mobilities at pW 7.0. 



ACKNOWLEDGMENT 



We wish to acknowledge the assistance of Mrs. F. Elizabeth White in the 

 growth and harvesting of the E. coli cultures and of Mr. Robert L. Sine in the 

 preparation of the electron micrograph. 



SUMMARY 



A dialyzable substance in extracts from E. coli has been found to inhibit 

 the decomposition of the RNP particles (previously referred to as the 40 S 

 component). This observation has led to the preparation of the RNP fraction 

 in a relatively high state of purity and stability. Physical measurements indi- 

 cate that the RNP component consists of uniformly spherical particles with 

 a molecular weight 1,240,000 and a diameter of 137 A. 



