TTA·TAA triplet repeats in plasmids form a non-H bonded structure

CTG·CAG, CGG·CCG, and AAG·CTT triplet repeats proximal to or in disease genes expand by a non-Mendelian genetic process to cause several human hereditary syndromes. As part of our physical, biological, and genetic studies on the 10 possible triplet repeats, we discovered that the TTA·TAA repeat, isolated from the upstream region of the variant surface glycoprotein gene of Trypanosoma brucei, shows a propensity to adopt a non-H bonded structure under appropriate conditions. The other nine triplet repeat sequences do not exhibit this property. (TTA·TAA)(n), where n = 90, 60, 30, and 18, cloned into pUC19 was studied by chemical and enzymatic probes as well as two-dimensional gel electrophoretic analyses under a variety of conditions. The helix opening was observed for all four inserts in supercoiled plasmids as a function of temperature, pH, metal ions, and buffer conditions using OsO₄, diethyl pyrocarbonate, and chloroacetaldehyde probes. This unusual property of the TTA·TAA repeat suggests that it plays a different role from the other nine triplet repeats in gene expression.