Cardiac myocyte differentiation induced by 3,5,3'-triiodo-L-thyronine (T3) in P19 teratocarcinoma cells is accompanied by preferential binding of RGG(T/A)CA direct repeats spaced by 4 base pairs in the DNA.

When treated with T3, P19 cells differentiate into ultrastructurally proven cardiac myocytes and express the cardiac ventricular specific marker ventricular myosin light chain 2V. This differentiation is irreversibly induced in culture during the first 48 hrs of exposure to T3. We studied the binding of P19-indigenous transcription factors of the Steroid-Thyroid-Retinoic superfamily of nuclear receptors to oligonucleotide response elements bearing direct, inverted and palindromic repeats of the consensus sequence RGG(T/A)CA. Electrophoretic mobility shift assays showed a preference in T3-treated P19 cells for binding RGG(T/A)CA "half sites" in direct repeat orientation separated by 4 base pairs. The specificity of binding was confirmed in competition experiments. This finding suggests that target genes bearing thyroid response elements spaced by 4 base pairs in their promoter regions play an important role in the cardiac differentiation induced by T3 in P19 teratocarcinoma cells.