CONFORMATIONAL PROPERTIES OF DNA STRANDS CONTAINING GUANINE-ADENINE AND THYMINE-ADENINE REPEATS

CD spectroscopy and PAGE were used to analyze cooperatively melting co nformers of DNA strands containing GA and TA dinucleotide repeats. The 20mer (GA)(10) formed a homoduplex in neutral solutions containing ph ysiological concentrations of salts and this homoduplex was not destab ilized even in the terminal (GA)(3) hexamers of (GA)(3)(TA)(4)(GA)(3), although the central (TA)(4) portion of this oligonucleotide preserve d the conformation adopted by (TA)(10). This observation demonstrates that homoduplexes of alternating GA and TA sequences can co-exist in a single DNA molecule, Another 20mer, (GATA)(5), adopted as a whole eit her the AT duplex, like (TA)(10), or the GA duplex, like (GA)(10), and switched between them reversibly. The concentration of salt controlle d the conformational switching. Hence, guanine and thymine share signi ficant properties regarding complementarity to adenine, while the TA a nd GA sequences can stack in at least two mutually compatible ways wit hin the DNA duplexes analyzed here. These properties extend our knowle dge of non-canonical structures of DNA.