Dimerization of the guanine-adenine repeat strands of DNA

Jovin and co-workers have demonstrated that DNA strands containing guanine- adenine repeats generate a parallel-stranded homoduplex, Here we propose th at the homoduplex is a dimer of the ordered single strand discovered by Fre sco and co-workers at acid pH. The Fresco single strand is shown here to be stabilized in aqueous ethanol where adenine is not protonated. Furthermore , we demonstrate that the strands dimerize at higher salt concentrations wi thout significantly changing their conformation, so that the dimerization i s non-cooperative. Hence, the Jovin homoduplex can form through a non-coope rative dimerization of two cooperatively melting single strands. The availa ble data indicate that the guanines stabilize the Fresco single strand wher eas the adenines cause dimerization owing to their known intercalation or c lustering tendency. The guanine-adenine repeat dimer seems to be a DNA anal og of the leucine zipper causing dimerization of proteins.