Retinoid X receptor alpha binds with the highest affinity to an imperfect direct repeat response element

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Retinoids regulate gene expression via retinoic acid receptors (RAR) and retinoid X receptors (RXR).
• RXR homodimers bind to specific DNA sequences, like direct repeat 1 (DR1), to activate gene expression in response to 9-cis-retinoic acid.
• The optimal DNA binding sequence for RXR homodimers remains incompletely understood.

Purpose of the Study:

• To identify the highest affinity DNA binding sequences for RXR alpha homodimers.
• To compare the functional potency of identified sequences with known response elements.
• To investigate the DNA binding specificity of RXR alpha homodimers at different positions within a response element.

Main Methods:

• A nonbiased strategy was employed to isolate high-affinity DNA sequences for RXR alpha homodimers from a random DNA pool.
• In vitro binding assays were used to determine DNA binding affinity.
• Transient transfection studies were conducted to assess the functional potency of identified DNA sequences as response elements.

Main Results:

• The imperfect DR1 sequence 5'-GGGGTCAAAGGTCA exhibited the highest in vitro binding affinity for RXR alpha homodimers.
• This identified sequence demonstrated greater potency as a response element compared to perfect DR1 sequences (AGGTCA or GGGGTCA).
• Differential DNA binding specificity was observed between the 5' and 3' half-sites for RXR alpha homodimers.

Conclusions:

• The sequence 5'-GGGGTCAAAGGTCA represents a high-affinity binding site for RXR alpha homodimers.
• DNA binding specificity is influenced by protein-protein interactions and the receptor's position on the response element, not solely by amino acid sequence.
• These findings enhance our understanding of retinoid receptor-mediated gene regulation.