Free Ribosomes

Figure 4.4. Schematic representation of a polyribosome.

the polypeptide only half the length of the complete one, and the ribosomes near the 5'-end contain only short peptides which have just started elongation. A structure in which the template polynucleotide is associated with many translating ribosomes is called the polyribosome.

The early electron microscopic observations in the mid-1950s demonstrated that ribonucleoprotein granules (ribosomes) are not dispersed uniformly in animal cell cytoplasm or in the preparations of microsome-derived particles but are clustered in groups. Evidence that such aggregates of ribosomes consist of particles that are connected by the mRNA chain and are engaged in translation was provided simultaneously by several groups (Gierer; Warner, Knopf & Rich; Wettstein, Staehelin & Noll; Penman, Scherrer, Becker & Darnell; Watson) in 1963. Polyribosomes were shown to be a form of actively translating ribosomes both in Eukaryotes and Prokaryotes.

The eukaryotic polyribosomes often look as ordered structures, rather then like beads on a

Figure 4.5. Electron micrograph showing predominantly circular, and sometimes spiral "G-like", polyribosomes on the rough endoplasmic reticulum of somatotrope cytoplasm from the rat pituitary. (Fig. 2 from A.K. Christensen, L.E. Kahn & C.M. Bourne, Amer. J. Anat. 178, 1-10, 1987; reproduced with permission).

randomly flexible thread. For example, most membrane-free polyribosomes from sea urchin eggs and embryos were visualized in the form of zig-zags (Martin & Miller, 1983), although linear forms were also present. The endoplasmic reticulum membrane-bound polyribosomes of protein-secreting cells are represented mostly by circular ("O-like") and spiral ("G-like") forms (Fig. 4.5). The functional significance of these distinctive arrangement patterns is not clear. In the case of circular polyribosomes it can be speculated that such a shape provides an efficient reinitiation of translation due to the proximity of the 5'-terminus to the termination codon: instead of the release of terminating ribosomes from mRNA they can recycle directly onto the 5'-end of the message to begin a new round of translation (see Chapters 15 and 17, and specifically Figs 15.14 and 17.12).

Under conditions of intensive protein synthesis, the distance between ribosomes along the mRNA chain within the polyribosome may be extremely short, and so the ribosomes may be packed together very tightly. This means that there may be roughly 50 nucleotide residues of the template per ribosome in the polyribosome. The implication here is that every 1 to 3 seconds, a ribosome finishes synthesizing the protein molecule near the 3'-end of the mRNA coding section and then jumps off the template or reinitiates; correspondingly, one new ribosome will become associated with the template at its 5'-end and will start moving toward the 3'-terminus. In more common cases about 100 nucleotide residues of mRNA per ribosome have been estimated. Also gaps and tails non-covered by ribosomes can be sometimes visualized along mRNA in polyribosomes; they may reflect the existence of some barriers inducing temporary stops (pauses) during elongation and the presence of untranslatable terminal sequences.

The existence of polyribosomes as a form of translating ribosomes in the cell explains the observation that ribosomes are abundant in the cell while the amount of mRNA is low. Indeed, ribosomal RNA accounts for about 80% of the total cellular RNA, whereas the mRNA content does not, as a rule, exceed 5%. This is easily understandable if one takes into account that the translation machinery of the cell is organized on the basis of polyribosomes: one mRNA is translated by many ribosomes, and one part of the translatable mRNA corresponds to 100 to 200 parts of ribosomal RNA by weight.

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