| Abstract: | Poly(pyrimidine) . poly(purine) tracts have been discovered in the 5'-flanking regions of many eucaryotic genes. They may be involved in the regulation of expression since they can be mapped to the nuclease-sensitive sites of active chromatin. We have found that poly(pyrimidine) . poly(purine) DNAs which contain 5-methylcytosine (e.g. polyd(Tm5C)] . polyd(GA)]) will form a triplex at a pH below 8. In contrast, the unmethylated analogue, polyd(TC)] . polyd(GA)] only forms a triplex at pHs below 6. Synthetic DNAs containing repeating trinucleotides and polyd(Um5C)] . polyd(GA)] behave in a similar manner. Thus the stability of a triplex can be controlled by methylation of cytosine. This suggests a model for the regulation of expression based upon specific triplex formation on the 5'-side of eucaryotic genes. |
