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RegA protein specifically binds to the region just downstream of the Shine-Dalgarno sequence and covers the initiation codon and the adjacent four codons. As a result it prevents the formation of ternary initiation complex.

Bacteriophage T4 DNA polymerase proved to be the repressor of its own synthesis: it binds to the initiation region and blocks the translation of the T4 DNA polymerase message. Thus the synthesis of the enzyme is regulated autogenously. This repressor covers the Shine-Dalgarno sequence and a section downstream, but not the initiation codon. It is likely that the repressor inhibits the initiation at the stage of the association of the ribosomal 30S particle with mRNA.

Another T4 protein that is autogenously regulated at the level of translation is the so-called single-stranded DNA-binding protein, or the gene 32 protein. Its binding site on the mRNA, however, is removed from the RBS far upstream and contains a pseudoknot (positions -40 to -67). The binding of the protein molecule induces a co-operative polymerisation of many copies of the protein along an unstructured RNA stretch overlapping the RBS. As the concentration of the gene 32 protein increases, the polymer of multiple copies of the protein reaches the Shine-Dalgarno sequence and covers it resulting in the repression of translation initiation.

16.5. Antisense Blockade

Ribosome binding site (RBS) of mRNA may be blocked by the interaction with a complementary RNA, called antisense RNA (for reviews, see Inouye, 1988; Wagner & Simons, 1994). This phenomenon is analogous to the translational repression. Several cases of such a blockade of RBS by a natural antisense RNA are known in bacteria. It seems that transcripts of accessory genetic elements, such as plasmids and transposons, as well as bacteriophages, are more often controlled by antisense RNAs than genomic mRNAs.

The best known example of natural antisense RNA of E. coli controlling a genomic mRNA translation is the so-called micFRNA (Mizuno et al., 1984; Aiba et al., 1987). There are two subspecies of this RNA, 93 and 174 nucleotides long respectively. The RNA regulates the synthesis of OmpF, a protein of the outer bacterial membrane that forms diffusion pores for small molecules. The antisense RNA (both r1^

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