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Figure 16.12. Two conformations of the translational operator (leader sequence with the translation start sequence) of the mRNA coding for ribosomal protein S15. The two-hairpin conformation (A) seems to be inactive in ribosome binding and initiation because of the closure of the RBS. The spontaneous transition into the pseudoknot conformation (B), possibly stimulated by initiating ribosomal particles, opens the RBS. After initiation the ribosomes exit from the pseudoknot due to the equilibrium between the conformations. Protein S15 has an affinity to the pseudoknot conformation (B), shifts the equilibrium towards it and stabilises the pseudoknot. As a result, the initiating ribosome is found to be trapped by the pseudoknot structure, and thus the translation is repressed. (Reproduced, with modifications, from C. Ehresmann, C Philippe, E. Westhof, L. Benard, C. Portier & B. Ehresmann, in "Frontiers in Translation ", A.T. Matheson, J.E. Davies, P.P. Dennis & W.E. Hill, eds., (Biochem. Cell Biol. 73), p.p. 1131-1140, 1995, with permission).

course of translational repression, has been confirmed by another series of facts. It has been demonstrated that ribosomal RNA added to the translation system removes the repression exerted by a corresponding ribosomal protein. Thus, in experiments in vitro the repressory action of protein L1 upon the synthesis of proteins L1 and L11, as well as the inhibition of the synthesis of proteins L10 and L7/L12 by the L10:(L7/ L12)4 complex, can be prevented specifically by adding ribosomal 23S RNA.

Proceeding from these observations, a model for the co-ordinated regulation of synthesis of ribosomal proteins can be proposed (Fig. 16.13) (see Nomura et al., 1980, 1982). The model is based on the idea that there is competition between ribosomal RNA and mRNA for binding to the core ribosomal proteins. Such proteins as S4, S7, S8, L1, L4, as well as the protein complex L10:(L7/L12)4, possess strong affinity to specific sites on ribosomal RNA. Therefore, after they have been synthesised, they become involved immediately in the assembly of ribosomal subunits through their direct binding to the 16S and 23S RNA. Intrinsic high affinity to ribosomal RNA and the cooperativity of ribosomal assembly involving other ribosomal proteins result in the sequestration of the newly formed ribosomal proteins in the course of the particle assembly. Under these conditions, mRNA molecules are unable to compete for the binding of these proteins and, therefore, do not associate with them; so they can be translated normally. However, when the number of ribosomal proteins increases compared to the amount of available ribosomal RNA, a free pool of such proteins is formed. This leads to the binding of the corresponding key proteins to their mRNAs, and the result is inhibited initiation and thus repressed translation. The strict sequential translation of polycistronic mRNA coding for a set of ribosomal proteins enables just one repressor protein and one site of its attachment for each mRNA to be sufficient for the co-ordinated control of translation of the whole set of proteins coded by a given mRNA. This simple mechanism provides a translation: r-protein synthesis translation: r-protein synthesis

Trna Repressor

repression

Figure 16.13. Model for ribosomal protein autoregulation. The newly synthesised ribosomal proteins bind to their sites on ribosomal RNA and assemble into ribosomal particles (the right part of the scheme). When ribosomal proteins are in excess, some of them bind to a target site (translational operator) on their own polycistronic mRNA (the lower part of the scheme).

direct regulatory relationship between the assembly of ribosomes and the synthesis of ribosomal proteins. 16.4.3.Translational Autoregulation of the Synthesis of Threonyl-tRNA

Synthetase

Threonyl-tRNA synthetase (ThrRSase) is coded by the first cistron, thrS, of the polycistronic message comprising also cistrons infC coding for IF3, rplT coding for the ribosomal protein L20, pheS and pheT coding for the two subunits of PheRSase, and himA coding for the protein called "host integration factor" (Springer & Grunberg-Manago, 1987). Translation of the thrS cistron has been shown to be repressed by the product of the translation, ThrRSase. The presence of an excess of the substrate, tRNAThr, abolishes the repressory action of the ThrRSase, and the enzyme is synthesised. Hence, only when the enzyme is in excess, it represses the translation of its own mRNA and thus stops its further production. Threonine starvation leading to the accumulation of deacylated tRNAThr derepresses the thrS mRNA. Under normal growth conditions the sequestration of aminoacylated tRNAThr in the complex with EF-Tu:GTP allows the free synthetase to repress the thrS mRNA translation.

ThrRSase has been demonstrated to bind directly to the region of thrS mRNA upstream of the initiation codon, adjacently to the RBS (Moine et al., 1988, 1990). The enzyme-repressor covers about 130 nucleotides. The most remarkable feature of the mRNA structure where Thr-tRNA binds as a repressor is that it mimics some elements of the structure of the tRNAThr, and specifically the structure of its

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