Conformations of three-stranded DNA structures formed in presence and in absence of the RecA protein

Using FTIR and UV spectroscopies, we have studied the structures of three-stranded DNA complexes (TSC) having two identical strands, containing all four bases, in parallel orientation. In the first system, an intermolecular TSC is formed by the addition of the third strand (ssDNA) previously coated with RecA protein to an hairpin duplex (dsDNA), in presence of ATP gamma S. In the second one, the formation of an intramolecular triplex is forced by folding back twice on itself an oligonucleotide. The sequences of the three strands are the same in both systems. The formation of the RecA-TSC, which accommodates all four bases, is evidenced by gel retardation assay, and by its biphasic melting profile observed by UV spectroscopy. Using FTIR spectroscopy, N-type sugars are detected in this structure. This shows that in the RecA-TSC studied in presence of the protein, the nucleic acid part adopts an extended form, in agreement with the model proposed by Zhurkin et al. (1,2) and electron microscopy observations (3-6). In contrast, the RecA-free intramolecular triplex in a non extended form has S-type sugars.