subspecies) is transcribed from a special chromosomal site removed from the ompF gene. It is induced by an increased osmolality of the medium. This RNA is partially complementary to the RBS of the ompF mRNA including the Shine-Dalgarno sequence and the initiator codon (Fig. 16.15). Thus the production of the micF RNA induced by high osmolality blocks the translation initiation of ompF mRNA due to their complementary interaction at the ompF RNA RBS. The result is the inhibition of the OmpF production in bacteria under these conditions.

The synthesis of the plasmid replication initiator protein RepA encoded in the enterobacterial plasmid R1 is also controlled by an antisense RNA, but in a more complicated way. Translation of the repA mRNA is tightly coupled with the translation of the preceding open reading frame coding for a short leader polypeptide Tap: translation of the tap cistron is necessary for unwinding of a stable hairpin that prevents an independent repA initiation (see Section 16.3.1). The antisense RNA called CopA (about 90 nucleotides long) is constitutively transcribed from the same plasmid and complementarily interacts with the region immediately upstream of the Shine-Dalgarno sequence of the tap cistron. The Shine-Dalgarno sequence itself and the downstream sequence including the initiation codon of the tap message are not involved in the complementary interaction. Nevertheless, the initiation of the tap message translation is blocked thus resulting in the inability of the initiation of the repA mRNA translation (Malmgren et al., 1996).

The synthesis of the transposase encoded by the insertion sequence IS10 of transposon Tn10 is controlled by an antisense RNA, called OUT RNA (70 nucleotides long) (Ma & Simons, 1990). In this case, however, it is produced by the transcription of the RBS region of the transposase gene itself, but in the opposite orientation to the transposase mRNA transcription (Fig. 16.16, see promoters pIN and pOUT). Hence, the OUT RNA is fully complementary to the region of the transposase mRNA (IN RNA) containing its RBS and thus directly blocks the binding of ribosomes and initiation of the mRNA translation. Similar regulation of mRNA translation by antisense RNAs transcribed from the target genes in the opposite orientation has been reported also for some bacteriophage and plasmid mRNAs.

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