On the basis of the structure of DNA-psoralen bis adducts (formed by psoralen with two thymines on opposite strands), a psoralen-oligonucleotide conjugate was designed to photoinduce a cross-link between the two DNA strands at a specific sequence. Psoralen was attached via its C-5 position to a 5'-thiophosphate group of an 11-mer homopyrimidine oligonucleotide. The 11-mer binds to an 11-base-pair homopurinehomopyrimidine sequence of a DNA fragment, where it forms a triple helix. Upon near-UV-irradiation, the two strands of DNA are crosslinked at the TpA step present at the triplex-duplex junction. The reaction is specific for the homopurine'homopyrimidine DNA sequence and requires both oligonucleotide recognition of the DNA major groove and intercalation of psoralen at the triplex-duplex junction. The yield of the photoinduced cross-linking reaction is quite high (>80%). Such psoralen-oligonucleotide conjugates are probes of sequencespecific triple-helix formation and could be used to selectively control gene expression or to induce site-directed mutations.
A photocrosslinking reagent (p-azidophenacyl) was covalently linked to an octathymidylate synthesized with either the natural ((3) anomer of thymidine or the synthetic (a) anomer. The oligothymidylate was further substituted by an acridine derivative to stabilize the hybrid formed with a complementary octadeoxyadenylate sequence via intercalation. A single-stranded 27-mer containing a (dA)8 sequence and a 27-mer duplex containing a (dA-dT)8 sequence were used as targets. Upon UV irradiation, photocrosslinking of the octathymidylate to its target sequence was observed, generating bands that migrated more slowly in denaturing gels. In the 27-mer duplex, both strands were photocrosslinked to the octathymidylate. Upon alkaline treatment of the irradiated samples, cleavage of the 27-mers was observed at specific sites. These reactions were analyzed at different salt concentrations. The location of the cleavage sites allowed us to demonstrate the following. (i) Both a and (3 oligothymidylates can recognize a DNA double helix containing an oligo(dA)-oligo(dT) sequence; the oligothymidylate binds to the major groove of DNA in a parallel orientation with respect to the adenine-containing strand of the DNA double helix. (ii) a oligothymidylates form helices with a complementary singlestranded oligodeoxyadenylate; the two strands have a parallel orientation independently of whether or not an intercalating agent is attached to the oligothymidylate. (iii) At low salt concentration, (3 oligothymidylates form a double helix with an oligodeoxyadenylate in which, as expected, the two strands are antiparallel; at high salt concentration, a triple helix is formed in which the second oligothymidylate is oriented parallel to the adenine-containing strand. These results show that it is possible to recognize an oligopurine-oligopyrimidine sequence in a DNA double helix via local triple-helix formation and to target photochemical reactions to specific sequences in both double-stranded and single-stranded nucleic acids.
An oligodeoxyribonucleotide, d(GCTCACAAT-X-ATTGTGAGC), where X represents a hexaethylene glycol chain, was studied using circular dichroism spectroscopy. Its conformation and conformational stability were compared to those of compounds where X was replaced by four thymines and to the duplex of same sequence without loop. The compound with the hexaethylene glycol chain can form a hairpin looped structure as well as a bulged duplex structure. In both cases the duplex region of the oligodeoxyribonucleotide exhibits the same conformation. In similar conditions the oligodeoxyribonucleotide with a four thymines loop forms exclusively a hairpin structure. Comparison between the thermodynamic parameters (delta H, delta S, delta G) associated with the formation of the structure of the three compounds are presented. In the case of the compound with the hexaethylene glycol chain it is shown that the large increase in its melting temperature (by about 35 degrees C in our experimental conditions) when compared to the non looped structure is mainly due to the fact that its melting process is intramolecular (monomolecular) whereas the other one is bimolecular.
Homopyrimidine oligodeoxynucleotides recognize the major groove of the DNA double helix at homopurine-homopyrimidine sequences by forming local triple helices. Phenanthroline was covalently attached to the 5' end of an 1i-mer homopyrimidine oligonucleotide of sequence d(TT-TCCTCCTCT). Simian virus 40 DNA, which contains a single target site for this oligonucleotide, was used as a substrate for the phenanthroline-oligonucleotide conjugate. In the presence of copper ions and a reducing agent, a single specific doublestrand cleavage site was observed at 20'C by agarose gel electrophoresis. The efficiency of double-strand cleavage was >70% at 20'C and pH 7.4. Secondary cleavage sites were observed when binding of the oligonucleotide to mismatched sequences was allowed to take place at low temperature. The exact location of the cleavage sites was determined by polyacrylamide gel electrophoresis of denatured fragments by using both simian virus 40 DNA and a synthetic DNA fragment containing the target sequence. The asymmetric distribution of the cleavage sites on the two strands revealed that the cleavage reaction took place in the minor groove even though the phenanthroline linker was located in the major groove. Linkers of different lengths were used to tether phenanthroline to the oligonucleotide and their relative efficacies of DNA cleavage were compared. Based on these comparative studies and on model building, it is proposed that the phenanthroline ring carried by the oligonucleotide intercalates from the major groove and that copper chelation locks the complex in place from within the minor groove where the cleavage reaction occurs.
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