Translin is a human single-stranded DNA and RNA binding protein that has been highly conserved in eukaryotic evolution. It consists of eight subunits having a highly helical secondary structure that assemble into a ring. The DNA and the RNA are bound inside the ring. Recently, some of us demonstrated that the human translin specifically binds the single-stranded microsatellite repeats, d(GT)(n), the human telomeric repeats, d(TTAGGG)(n), and the Tetrahymena telomeric repeats, d(GGGGTT)(n). These data suggested that translin might be involved in recombination at d(GT)(n).d(AC)(n) microsatellites and in telomere metabolism. Other data indicated that translin might stimulate binding of telomerase to single-stranded telomeric overhangs by unwinding secondary structures formed by the telomeric repeats. Here we present a circular dichroism (CD) analysis of complexes formed between the human translin and the microsatellite and telomeric oligodeoxynucleotides d(GT)(12) and d(TTAGGG)(5). We report that conformational changes occur in both the translin and the oligodeoxynucleotides upon formation of the complexes. In translin octamers bound to the oligodeoxynucleotide d(GT)(12), the fraction of alpha-helices decreases from approximately 67% to approximately 50%, while the fraction of turns and of the unordered structure increases from approximately 11% to approximately 17% and from approximately 19% to approximately 24%, respectively. In the bound oligodeoxynucleotide d(GT)(12), we observed CD shifts which are consistent with a decrease of base stacking and a putative anti-syn switch of some guanines. The oligodeoxynucleotide d(TTAGGG)(5) formed intramolecular quadruplexes under the conditions of our assays and translin was found to unfold the quadruplexes into structures consisting of a single hairpin and three unwound single-stranded d(TTAGGG) repeats. We suggest that such unfolding could account for the stimulation of telomerase activity by translin mentioned above.