Size Appearance and Subdivision into Subunits

When examined by electron microscopy the isolated bacterial ribosomes at first approximation look like compact rounded particles with linear sizes of about 200 to 250 Â (Fig. 5.1), and somewhat larger, from 200 to 300 Â, in the case of eukaryotic ribosomes. Ribosomes from different organisms and cells, whether prokaryotic or eukaryotic ones, have a strikingly similar appearance.

A characteristic feature of one of the visible ribosomal projections is a groove dividing the ribosome into two unequal parts (Fig. 5.2). This subdivision reflects the fact that the ribosome consists of two separable subparticles, or ribosomal subunits. Under certain conditions, e.g., if the concentration of magnesium ions in the medium is sufficiently low, the ribosome dissociates into two subunits with a mass ratio of about 2:1 (Fig. 5.3). The prokaryotic 70S ribosome dissociates into subunits with the sedimentation coefficients 50S (molecular mass 1.65 x 106 daltons) and 30S (molecular mass 0.85 x 106 daltons):

The eukaryotic 80S ribosome dissociates into 60S and 40S subunits: 80S ® 60S + 40S

The dissociation can be also induced by Na+, Li+, and urea, as well as by high concentrations (above 0.5 M) of such "physiological" monovalent cations as K+ and NH4+. The dissociation of Escherichia coli 70S ribosomes following a decrease of the Mg2+ concentration is illustrated

Figure 5.1. Electron micrograph of the 70S ribosomes isolated from Escherichia coli. To achieve the contrast necessary for the particles to be seen in the electron microscope, the isolated 70S ribosomes are applied on an ultra-thin carbon film; the film with attached particles is treated by uranyl acetate solution and dried in air. The particles become embedded in uranyl acetate that fills cavities and grooves. The ribosomal particles having lower electron density than uranyl acetate appear negatively stained against the background of uranyl acetate. The arrows indicate the L7/L12 stalk described in the text. (Original photo by V. D. Vasiliev).

Figure 5.2. Electron micrographs of individual Escherichia coli 70S ribosomes illustrating their subdivision into two unequal subunits; the images are oriented such that the small subunit is at the top and the large subunit is at the bottom. (Original photos by V.D. Vasiliev; see also C.E. Hall & H.S. Slater, J. Mol. Biol. 1, 329-332, 1959; H.E. Huxley & G. Zubay, J. Mol. Biol. 2, 10-18, 1960; V.D. Vasiliev, FEBS Lett. 14, 203-205, 1971).

A: Ribosomes contrasted by metal shadowing. In this case, to achieve the necessary contrast the suspension of isolated 70S ribosomes is applied to a carbon film surface and freeze-dried; the particles are shadowed by metal (tungsten or tungsten-rhenium alloy) using vacuum evaporation at an angle of about 75° to film surface; this yields shadow-cast particles.

B: Negatively stained ribosomes, prepared as described in the legend to Fig. 5.1.

Figure 5.2. Electron micrographs of individual Escherichia coli 70S ribosomes illustrating their subdivision into two unequal subunits; the images are oriented such that the small subunit is at the top and the large subunit is at the bottom. (Original photos by V.D. Vasiliev; see also C.E. Hall & H.S. Slater, J. Mol. Biol. 1, 329-332, 1959; H.E. Huxley & G. Zubay, J. Mol. Biol. 2, 10-18, 1960; V.D. Vasiliev, FEBS Lett. 14, 203-205, 1971).

A: Ribosomes contrasted by metal shadowing. In this case, to achieve the necessary contrast the suspension of isolated 70S ribosomes is applied to a carbon film surface and freeze-dried; the particles are shadowed by metal (tungsten or tungsten-rhenium alloy) using vacuum evaporation at an angle of about 75° to film surface; this yields shadow-cast particles.

B: Negatively stained ribosomes, prepared as described in the legend to Fig. 5.1.

Figure 5.3. Sedimentation pattern (analytical ultracentrifugation with schlieren optic) of the E. coli 70S ribosomes and the products of their dissociation achieved by lowering the Mg2+ concentration in the medium. A: 70S ribosomes in 10 mM MgCl2, 100 mM NH4Cl.

B: 30S and 50S subunits in 1 mM MgCl2, 100 mM NH4Cl.

Figure 5.3. Sedimentation pattern (analytical ultracentrifugation with schlieren optic) of the E. coli 70S ribosomes and the products of their dissociation achieved by lowering the Mg2+ concentration in the medium. A: 70S ribosomes in 10 mM MgCl2, 100 mM NH4Cl.

B: 30S and 50S subunits in 1 mM MgCl2, 100 mM NH4Cl.

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