Effect of monovalent cation-induced telomeric DNA structure on the binding of Oxytricha telomeric protein.

Oligonucleotides bearing 4 repeats of telomeric deoxyguanosine-rich sequence undergo a monovalent cation-induced transition to a folded conformation with G-G base pairs, modeled as a 'G-quartet' structure. We have now deduced the rates of folding and unfolding of d(TTTTGGGG)4, which has four repeats of the Oxytricha telomeric DNA sequence. The estimated average values of delta G for the folded form at 37 degrees C are -2.2 kcal/mol and -4.7 kcal/mol in 50 mM na+ and K+, respectively. The fully folded DNA is not recognized by the Oxytricha telomere-binding protein; the substrate for protein binding has properties consistent with its being partly or fully unfolded. In confirmation of this conclusion, prevention of DNA folding by methylation enables the protein to bind as rapidly in the presence of monovalent cations as in their absence. The slow unfolding (t1/2 = 4 hr and 18 hr at 37 degrees C in Na+ and K+, respectively) of the DNA suggests that such structures would be long-lived if they formed in vivo, unless they can be actively unfolded. The inability of the telomere-binding protein to bind the stable, folded form of the 4-repeat telomeric sequence is a problem that may be circumvented in vivo by avoiding four single-stranded repeats.