Figure 16.14. Secondary structure of the operator region of the thrS mRNA. This structure has an affinity to ThrRSase that serves as a translational repressor for the its own mRNA (thrS mRNA). The elements identical to those in anticodon and acceptor helices of tRNAThr are marked by grey boxes. The SD sequence and initiation codon of the thrS mRNA are in empty and black boxes, respectively. (H. Moine, P. Romby, M. Springer, M. Grunberg-Manago, J.-P. Ebel, C. Ehresmann & B. Ehresmann, Proc. Natl. Acad. Sci. USA 85, 7892-7896, 1988; J. Mol. Biol. 216, 299-310, 1990).

anticodon stem-loop which is recognised by the enzyme. The predicted and experimentally tested secondary structure of the operator of the thrS mRNA is presented in Fig. 16.14. It is characterised by two compound hairpins. The hairpin adjacent to the Shine-Dalgarno sequence contains a seven-nucleotide end loop strongly resembling the anticodon loop of tRNAThr, including the anticodon-like triplet CGU in the middle. The five base pairs helix underlying the loop is also similar to the anticodon helix of the tRNAThr.(see Fig. 16.14, upper insert). It is this stem-loop element of the thrS mRNA operator that is specifically recognised and strongly bound by the ThrRSase. In addition, the other compound hairpin has also been found to participate in the binding of the enzyme-repressor to the thrS mRNA operator. Its basal part (the upper 7 base pairs in Fig. 16.14) seems to resemble the acceptor stem of tRNAThr, with the ACCA continuation at the 3' side (Fig. 16.14 lower insert). The binding of the ThrRSase to these recognition elements mimicking those of the tRNAThr results in blocking the initiation of translation of the thrS mRNA by preventing the binding of the ribosomal particles to the RBS. It is likely that the binding the enzyme-repressor stabilises the complementary interaction, otherwise weak, of the Shine-Dalgarno sequence UAAGGA with the sequence UUUUUA in positions -51 to -56, and hence makes the RBS inaccessible for ribosomal particles (Brunel et al., 1995).

Since tRNAThr and the thrS operator are recognised by the ThrRSase in the analogous way, the excess tRNAThr is able to displace the mRNA from the enzyme. Therefore, the tRNAThr acts as an antirepressor, this implying that the level of free tRNA can modulate the repressor activity of the ThrRSase. The repression/derepression control of thrS mRNA translation ensures precise adjustment of the ThrRSase synthesis depending on the changes of tRNAThr and ThrRSase levels in the cell.

16.4.4.Regulation of Translation of Bacteriophage T4 mRNAs

Several translational repressors govern the expression of different bacteriophage T4 mRNAs (reviewed by Gold, 1988). RegA protein represses the translation of a large number of early T4 messages. One of such messages is the T4 rIIB mRNA; its RBS includes the sequence as follows:

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