Specific Gene Regulation by Oligodeoxynucleotides Covalently Linked to Intercalating Agents

Abstract

In living cells, the regulation of gene expression occurs at different levels: transcription of one of the DNA strands into RNA, splicing of pre-messenger RNAs which eliminate intron sequences after transcription of mosaic genes, post-transcriptional modifications of mRNAs (capping, polyadenylation...), transfer of mRNA from the nucleus to the cytoplasm, translation of mRNA, post-translational modifications of proteins... In most cases the regulation is achieved by proteins which bind either DNA sequences to block or activate transcription or RNA to block, e.g., translation (see Hélène and Lancelot, 1982, for a review). More recently it has been shown that, in bacteria, small RNAs could play a regulatory role similar to that of regulatory proteins by hybridizing to mRNAs (see Green et al., 1986, for a review). The control of plasmid copy number and immunity in bacteria is also achieved by a small RNA (RNA I) which arises from transcription in the reverse direction as compared to that of the primer RNA (RNA II) utilized to initiate plasmid replication. Being transcribed from opposite strands the two RNAs are fully complementary and their association prevents replication initiation (see Tomizawa, 1986 and references therein). The role of RNAs in the regulation of mRNA translation was first demonstrated in E.coli. The translation of mRNAs for proteins involved in the regulation of plasmid R1 replication is controlled by a small RNA which is transcribed in the reverse direction with respect to the mRNA. Complete base pairing occurs between the mRNA and the regulatory RNA which prevents ribosome from translating the mRNA (Light and Molin, 1983). A similar situation was observed for the regulation of transposase synthesis involved in Tn 10 transposition (Simons et al., 1983). It was then discovered that the synthesis of the ompF protein was controlled by a small RNA transcribed from a DNA region close to the ompC gene. Complex formation between the regulatory RNA and the ompF mRNA did not involve complete base pairing but ensured sufficient stability to interfere with translation (Mizuno et al., 1984). A more recent study showed that hybrid formation between a small RNA and the 5′- end of a mRNA could induce premature termination of transcription, thereby controlling gene expression at another level than translation (Okamoto and Freundlich, 1986). All the examples briefly described above involve bacterial systems. There is some evidence that a similar involvement of RNAs as regulatory elements could occur in eukaryotic cells as well (Heywood, 1986; Spencer et al., 1986; Williams and Fried, 1986).