The interplay of half-site sequence and spacing on the activity of direct repeat thyroid hormone response elements.

Direct repeats of the hexamer AGGTCA can serve as response elements for vitamin D, thyroid hormone, or retinoic acid. The specificity of the response appears to reside in the spacing between the hexamers, with response elements for vitamin D restricted to direct repeats separated by a 3-base pair (bp) spacer, thyroid hormone a 4-bp spacer, and retinoic acid a 5-bp spacer (3-4-5 rule). Recently we have shown that the optimum thyroid hormone receptor binding site consists of an 8-bp sequence (TAAGGTCA), not a hexamer. Therefore we tested whether the 3-4-5 rule is valid for octamer sequence direct repeats. In transfection experiments octamer direct repeats with 3-, 4-, or 5-bp spacers conferred equivalently strong thyroid hormone responses, although a repeat with a 9-bp spacer was substantially weaker. For the 4- and 5-bp spacer constructs, the 5' half-site octamer had as strong an influence on thyroid hormone induction as did the 3' half-site octamer, although for the 3-bp spacer construct the 5' octamer was marginally less potent than the 3' octamer. Transfection and gel shift experiments did not suggest a simple correlation between the binding of thyroid hormone receptor-retinoid X receptor heterodimers and thyroid hormone induction from these response elements. We conclude that half-site sequence can override the effect of spacing in determining the hormone responsiveness of a direct repeat response element. In addition, the thyroid hormone response may not be due simply to the binding of thyroid hormone receptor-retinoid X receptor heterodimers to the DNA.