S

Figure 5.4. Scheme of ribosome dissociation into subunits. Some factors inducing the dissociation and promoting the reassociation are indicated.

by the sedimentation patterns shown in 5.3.

The dissociation is reversible. The restoration of a proper Mg2+ concentration and the removal of dissociating agents result in reassociation of ribosomes. The reassociation is also promoted by Ca2+, diamines and polyamines, and alcohols. Some factors contributing to and counteracting the dissociation of ribosomes are indicated in Fig. 5.4.

After dissociation, the ribosomal subunits can be separated in the preparative ultracentrifuge, and then studied individually. A unique asymmetrical shape of each of them has been detected and is described below.

5.2. Small Subunit

Different electron microscopic projections of the bacterial (Escherichia coli) ribosomal 30S subunit and the corresponding crude morphological model are shown in Fig. 5.5. The 30S subunit is somewhat elongated, and its length is about 230 A. The subunit may be subdivided into lobes which are referred to as the "head" (H), "body" (B), and "side bulge" or "platform" (SB). The groove separating the head from the body is quite distinct.

The eukaryotic 40S subunit has a similar morphology, although two additional details of structure may be mentioned. The first is a protuberance, or "eukaryotic bill" on the head. Second, the end of the body distal to the head appears to be bifurcated due to the presence of some additional mass; this bifurcation is referred to as the "eukaryotic lobes" (Fig. 5.6).

More reliable information about ribosomal subunits can be derived from electron microphotographs if averaged images rather than individual ones are examined. Averaging allows the statistical noise on electron microphotographs to be eliminated. This contributes towards a better visualization of the common features in the images of a given particle type. For such an averaging a set of particle images in the same projection is digitized using microdensitometer and processed with computer. The images are aligned precisely with respect to each other and then summed together to give an "average" image. All non-reproducible details of original images such as resulted from variations of stain distribution around the particle, radiationinduced structural alterations, variations in the background support film, are removed, leaving the common elements remained on the averaged image. Examples of such an averaging for negatively stained 30S subunits of Escherichia coli are given in Fig. 5.7 A. All three projections show that the head is separated from the remainder of the subunit by a distinct deep groove, the 'neck' being rather thin. An example of the averaging for negatively stained 40S subunits of rat liver ribosomes is presented in Fig. 5.7 B. Again, the "neck" is thin, and the bill of the head can be seen clearly in two of the

Figure 5.5. Electron micrographs of individual 30S ribosomal subunits of E. coli and a model of them in three projections (V.D. Vasiliev, Acta Biol. Med. Germ. 33, 779-793, 1974). The upper two rows show metal-shadowed particles, prepared as described in the legend to Fig 5.2 A. The next two rows show uranyl acetate-stained particles, prepared as described in the legend to Fig. 5.1. The lower row is the model. The left column is the images of the 30S subunit and its model in the projection when it is viewed from the side opposite to that facing the 50S subunit in the complete ribosome. The middle column is the images of the 30S subunit and its model in the narrow side (frontal) projection. The right column is the images of the 30S subunit and its model in the projection when it is viewed from the side facing the 50S subunit in the ribosome. (Original photos by V.D. Vasiliev).

Rel Empreinte Digitale

Figure 5.6. Electron micrographs of the individual 40S subunits of rat liver ribosomes and a model of them in three projections (V. D. Vasiliev, O. M. Selivanova, G. Lutsch, P. Westermann, & H. Bielka, FEBS Letters 248: 92-96, 1989). The upper two rows show metal-shadowed particles, prepared as described in the legend to Fig. 5.2 A. The next two rows show uranyl acetate-stained particles, prepared as described in the legend to Fig. 5.1. The lower row is the model. Three columns of images are the 40S subunit and its model in the same projections as those for the 30S subunit shown in Fig. 5.3. (Original photos by V.D. Vasiliev).

Figure 5.6. Electron micrographs of the individual 40S subunits of rat liver ribosomes and a model of them in three projections (V. D. Vasiliev, O. M. Selivanova, G. Lutsch, P. Westermann, & H. Bielka, FEBS Letters 248: 92-96, 1989). The upper two rows show metal-shadowed particles, prepared as described in the legend to Fig. 5.2 A. The next two rows show uranyl acetate-stained particles, prepared as described in the legend to Fig. 5.1. The lower row is the model. Three columns of images are the 40S subunit and its model in the same projections as those for the 30S subunit shown in Fig. 5.3. (Original photos by V.D. Vasiliev).

projections; the bifurcated tail, or eukaryotic lobes, are also prominent.

It should be noted that the small ribosomal 30S subunit of archaebacteria (archaea) has a morphology which is intermediate between that of the eubacterial 30S subunit and the eukaryotic 40S subunit: the archaebacterial subunit has a characteristic bill on the head but does not possess the eukaryotic lobes at the end of the body.

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