The role of polyamines, Na+ and K+ in the formation of triple helices between purine oligonucleotides and the promoter region of the human c-src proto-oncogene

Binding constants for tripler formation between purine-rich oligonucleotide s and a pyrimidine.purine tract of the human c-src proto-oncogene were meas ured by fluorescence polarization in the presence of polyamines, Na+ and K. In both the hexamine and tetramine series, the longer polyamines had the larger binding constants for tripler formation at low concentrations of pol yamine. At higher concentrations all values tended to plateau in the 10(9)/ M range. In contrast to previous reports, K+ did not inhibit tripler format ion and at 150 mM the binding constants were again in the 10(9)/M range for both an 11mer and 22mer oligonucleotide. At 150 mM K+ the addition of poly amines did not lead to any significant increase in the binding constants, I t was determined that the lack of inhibition by K+ was due to the low conce ntration (1 nM) of purine oligonucleotide required for the fluorescence pol arization technique, At higher concentrations (1 mu M) self-association of the oligonucleotide was observed, These results suggest that in vivo, at le ast for the c-src promoter, the inhibition of tripler formation by K+ may n ot be detrimental. However, it may be difficult to achieve binding constant s above similar to 10(9)/M even in the presence of polycations.