Poly(dG).poly(dC) at neutral and alkaline pH: the formation of triple stranded poly(dG).poly(dG). poly(dC)

Abstract

Alkaline titrations of different samples of poly(dG).poly(dC) and of the constituent homopolymers poly(dG) and poly(dC) have been performed in 0.15 M NaCl and their CD spectra followed. Sample I contained a slight excess of poly(dC) (52% C: 48% G) and showed a single reversible transition (pK = 11.9) due to the dissociation of double stranded poly(dG).poly(dC). Sample II, containing an excess of poly(dG) (43% C: 57% G), showed two transitions (pK1 = 11.4, PK2 = 11.9) the first one being only partially reversible. Examination of the CD spectra along the alkaline titrations indicated the presence of another hydrogen-bonded complex of higher G content. Mixing curves performed at pH 8 have confirmed the presence of a 2G: 1C complex, besides the double stranded complex. It can be formed in amounts up to 30% by mixing the two homopolymers, alkali treatment and heating. The CD spectra of the two complexes have been computed from the CD data of the mixing curves. This permitted the determination of the concentrations of both complexes and homopolymers in all samples. The ratio of triple to double stranded complex is not only dependent on the G/C ratio of the sample, but also a function of the previous physico-chemical conditions. These results explain the variability of many properties of different poly(dG).poly(dC) samples observed by other workers.

Characterization of the DNA triplex formed by d(TGGGTGGGTGGTTGGGTGGG) and a critical R · Y sequence located in the promoter of the murine Ki‐ras proto‐oncogene

“…Interestingly, an intense and negative ellipticity at 212 nm is induced upon triplex formation, as previously observed for other triplex structures [5,6,24]. Another outstanding feature present in the 20GT: 30RY spectrum is the presence of a negative ellipticity at 277 nm, which according to a study on poly(dG).poly(dC) has been attributed to the formation of triplex helices containing G. GC triads [20,21,32,33]. …”

“…The first evidence about the formation of R. RY triplexes was obtained with mixtures of poly(G) plus poly(C) [20,21] and poly(U) plus poly(A) [22]. Thereafter, Kohwi and KowhiShigematzu demonstrated that a segment of poly dG.…”

Protonated polynucleotides structures - 23. The acid-base hysteresis of poly(dG).poly(dC)

“…The hysteresis of poly(dG)-poly(dC) is complicated by the f6rmation of an acid complex at the beginning of the acid titration . Furthermore, we have shown that most samples of poly(dG)*poly(dC) contained two hetero-complexes, 8 poly(dG)*po1y(dC) and poly(dG).poly(dG)*poly(dC) The distribution of these C Information Retrieval Umited I Falconbwg Court London W1V 5FG England…”

Triple helix formation inhibits transcription elongation in vitro.

“…Two structural motifs by which these triple helices form have been described. The more widely studied motif is based on Hoogsteen hydrogen bonding of a pyrimidine oligonucleotide wherein T recognizes A-T base pairs and protonated C (C+) recognizes G-C base pairs (1)(2)(3)(4)(5)(6). The ODNs bind in the major groove of DNA such that the pyrimidine ODN is parallel to the purine strand of the duplex.…”