A TRIPLE-HELIX OBTAINED BY SPECIFIC RECOG NITION OF ALL 4 BASES IN DUPLEX DNA CAN ADOPT A COLLAPSED OR AN EXTENDED FORM

It has been proposed that during homologous recombination promoted by RecA DNA triple helices can be formed between a Watson Crick duplex an d a homologous third strand without any sequence constraint A triple h elix, obtained by targeting the d(AGTTAGCATG) sequence containing all 4 bases, in which both homologous strands are oriented in a parallel d irection with respect to each other stabilized by addition of Mn2+ ion s has been studied by UV and FTIR spectrosco pies. We have characteriz ed the sugar conformations of this tripler. All strands are found to c ontain S type sugars (C2'endo, B family form). Progressive addition of propidium iodide induces a complete reorientation of the sugar geomet ry to a N type conformation (C3'endo, A family form). This sugar repuc kering is consistent with a conformational transition from a collapsed to an extended DNA triple helical structure.