U • G

Figure 16.8. Comparison of the predicted secondary structure of the polycistronic mRNA region (L24 - L5 junction) recognised by ribosomal protein S8 acting as a translational repressor (left; initiation codon and SD sequence are boxed) and that of the 16S ribosomal RNA region recognised by protein S8 in ribosome assembly (right). Homologous helices are enclosed by the broken line. (P.O. Olins & M. Nomura, Nucleic Acids Res. 9, 1757-1764, 1981).

of ribosomal proteins on ribosomal RNA (see Nomura et al., 1980; Olins & Nomura, 1981).

As an example, Fig. 16.8 presents a comparison of the primary and predicted secondary structures of the intercistronic L24-L5 region and the origin of the L5 cistron assumed to be the binding site of the repressory protein S8 (Fig. 16.7 the third line), and the region of the ribosomal 16S RNA that binds protein S8. The homology is apparent. Another example shown in Fig. 16.9 is the structural homology between the L11 mRNA leader (Fig. 16.7 the fifth line) and the L1-protected region of the 23S ribosomal RNA (Yates & Nomura, 1981).

In all the above cases, the model of repressory action of the corresponding ribosomal protein seems to be analogous to the repression of S cistron by the phage MS2 or R17 coat protein (see above, Section 16.4.1): the RBS is involved in a rather labile secondary and tertiary structure which does not, by itself,

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