Oligodeoxyribonucleotides covalently linked to an intercalating agent via a polymethylene linker were synthesized. Oligothymidylates attached to an acridine dye (Acr) through the 3'-phosphate group [(Tp),(CH2)mAcr] specifically interact with the complementary sequence. The interaction is strongly stabilized by the intercalating agent. By using absorption and fluorescence spectroscopies, it is shown that complex formation between (Tp),(CH2)mAcr and poly(rA) involves the formation of n A-T base pairs, where n is the number of thy. mines in the oligonucleotide. The acridme ring intercalates between ANT base pairs. Fluorescence excitation spectra reveal the existence of two environments for the acridine ring, whose relative contributions depend on the linker length (m). The binding of (Tp)4(CH2)mAcr to poly(rA) is analyzed in terms of site binding and cooperative interactions between oligonucleotides along the polynucleotide lattice. Thermodynamic parameters show that the covalent attachment of the acridine ring strongly stabilizes the binding of the oligonucleotide to its complementary sequence. The stabilization depends on the linker length; the compound with m = 5 gives a more stable complex than that with m = 3. These results open the way to the synthesis of a family of molecules exhibiting both high-affinity and high-specificity for a nucleic acid base sequence.Molecules with high affinity and base-sequence specificity are required to control gene expression at different levels. Several possibilities may be contemplated depending on whether double-stranded or single-stranded nucleic acids are chosen as targets. For example, the regulation of transcription involves proteins that recognize specific duplex DNA sequences in both prokaryotes and eukaryotes (1, 2). However, there is no general rule yet established that could allow us to build an oligopeptide sequence aimed at recognizing a given base or base pair sequence (3). Moreover, the amino acid side chains that are involved in contacts with base or base pairs are not contiguous in the polypeptide chain.The most obvious candidate to allow for the specific recognition of a nucleic acid fragment is an oligonucleotide with the complementary sequence, provided the nucleic acid bases in the target sequence have their hydrogen bonding sites available. This is obviously so if the target nucleic acid region exists as a single-stranded structure. This might also be true in a duplex structure if a sufficient supplementary energy of interaction were provided either by modifying the oligonucleotide (see below) or by imposing constraints on the duplex structure (e.g., in superhelical DNA or upon binding of melting proteins). Also it must be kept in mind that important regulatory regions in transcription and replication must be transiently opened as observed-e.g., in the complex formed by bacterial RNA polymerase with a promoter (the so-called "open complex") (4) or at the replication fork (5). Actively transcribed genes in eukaryotes also possess regions upstream f...

The GreA and GreB proteins of Escherichia coli show a multitude of effects on transcription elongation in vitro, yet their physiological functions are poorly understood. Here, we investigated whether and how these factors in¯uence lateral oscillations of RNA polymerase (RNAP) in vivo, observed at a protein readblock. When RNAP is stalled within an (ATC/ TAG) n sequence, it appears to oscillate between an upstream and a downstream position on the template, 3 bp apart, with concomitant trimming of the transcript 3¢ terminus and its re-synthesis. Using a set of mutant E.coli strains, we show that the presence of GreA or GreB in the cell is essential to induce this trimming. We show further that in contrast to a ternary complex that is stabilized at the downstream position, the oscillating complex relies heavily on the GreA/GreB-induced`cleavage-and-restart' process to become catalytically competent. Clearly, by promoting transcript shortening and re-alignment of the catalytic register, the Gre factors function in vivo to rescue RNAP from being arrested at template positions where the lateral stability of the ternary complex is impaired.

New molecules with high and specific affinity for nucleic acid base sequences have been synthesized. They involve an oligodeoxynucleotide covalently attached to an intercalating dye. Visible absorption spectroscopy and fluorescence have been used to investigate the binding of poly(rA) to octadeoxythymidylates substituted by a 9‐aminoacridine derivative in different positions along the oligonucleotide chain. The 9‐amino group of the acridine dye was linked through a polymethylene bridge to the 3′‐phosphate, the 5′‐phosphate, the fourth internucleotidic phosphate or to both the 3′‐ and 5′‐phosphates. Different interactions of the acridine dye were exhibited by these different substituted oligodeoxynucleotides when they bind to poly(rA). The interaction was shown to be specific for adenine‐containing polynucleotides. The stability of these complexes was compared with that of oligodeoxynucleotides substituted by an alkyl group on the 3′‐phosphate. The increase in stability due to the presence of the intercalating dye has been determined from the comparison of melting temperatures. These results are discussed with respect to the strategy of synthesis of a new class of molecules with high affinity and high specificity for nucleic acid base sequences.

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