Size and Shape of Ribosomal Subunits

In order to place the ribosomal components in the appropriate topography, the size and shape of the ribosomal subunits must be defined. 

This has been done mainly using two techniques, electron microscopy and small-angle scattering. 

The 50 S subunit as seen by electron microscopists is an asymmetric, trinodal particle. Three protuberances—the so-called L7/L12 stalk, the nose, and the LI-shoulder—appear on a hemispherical body. The models proposed by various groups are shown in Fig. 5. 

For both the 30 S and 50 S subunits, additional neutron-scattering studies have been made to determine the distribution of the protein and the RNA within the subunits. It has been shown in both cases that the 

The 70 S ribosome itself, as depicted in Fig. 6, appears to be rather symmetric, with the 30 S subunit lying somewhat obliquely across the 50 S subunit with the head of the 30 S subunit near the LI shoulder on the 50 S subunit. The platform region of the 30 S subunit faces the 50 S subunit providing an interesting shielded area where the ribosomal function apparently occurs. It appears from electron micrographs that the structure of the subunits is not greatly altered upon their association to form the 70 S ribosome. 

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