CENTROMERIC DODECA-SATELLITE DNA-SEQUENCES FORM FOLDBACK STRUCTURES

The evolutionarily conserved centromeric dodeca-satellite DNA has an a symmetric distribution of guanine and cytosine residues resulting in o ne strand being relatively G-rich. This dodeca-satellite G-strand cont ains a GGGA-tract that is similar to the homopurine tracts found in mo st telomeric DNA sequences. Here, we show that the dodeca-satellite G- strand forms intramolecular hairpin structures that are stabilized by the formation of non-Watson-Crick G(.)A pairs as well as regular Watso n-Crick G(.)C pairs. Special stacking interactions are also likely to contribute significantly to the stability of this structure. This hair pin conformation melts at relatively high temperature, around 75 degre es C, and is detected under many different ionic and pH conditions. As judged by electron microscopy visualization, these structures can be formed in a B-DNA environment. Under the same experimental conditions, neither the C-strand nor the double-stranded dodeca-satellite DNA wer e found to form any unusual DNA structure. A protein activity has been detected that preferentially binds to the single-stranded dodeca-sate llite C-strand. The biological relevance of these results is discussed in view of the similarities to telomeric DNA.