The first problem regarding the localization of the functional sites of the ribosome has to do with whether they are assigned to one of the two ribosomal subunits, or to both subunits together. In the simplest case the experimental solution of this problem is as follows. Ribosomes are dissociated to yield large and small subunits, the subunits are separated, and the tested ligand is added to each of them (in the presence of a sufficient concentration of magnesium ions, which is required to observe any binding to the ribosome). It has been demonstrated in this type of experiment that the isolated bacterial 30S subunit binds the template polynucleotide whereas the 50S subunit does not (Takanami & Nakamoto, 1963). On the basis of this result, it is generally accepted that the mRNA-binding site of the ribosome is located only on the small (30S or 40S) subunit.

The isolated small ribosomal subunit protects an mRNA region of principally the same length as does the full ribosome, provided a tRNA is also bound with the subunit (Fig. 9.2 B). In the absence of tRNA, however, the 30S subunit protects about 40 nucleotides: the 3'-section of about dozen nucleotides long becomes less protected or unprotected (Fig. 9.2 C).

Several approaches have been used for identifying the ribosomal proteins and the ribosomal RNA regions that take part in the organization of the mRNA-binding site of the 30S ribosomal subunit. The

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