Abstract :
[en] Retinoic Acid Receptors (RARs) heterodimerise with Retinoid X Receptors (RXRs)
and bind to RA-response elements (RAREs) in the regulatory regions of their
target genes. While previous studies on limited sets of RA-regulated genes have
defined canonical RAREs as direct repeats of the consensus RGKTCA separated by 1,
2 or 5 nucleotides (DR1, DR2, DR5), we show that in mouse embryoid bodies or F9
embryonal carcinoma cells, RARs occupy a large repertoire of sites with DR0, DR8
and IR0 (inverted repeat 0) elements. Recombinant RAR-RXR binds these
non-canonical spacings in vitro with comparable affinities to DR2 and DR5. Most
DR8 elements comprise three half sites with DR2 and DR0 spacings. This specific
half site organisation constitutes a previously unrecognised, but frequent
signature of RAR binding elements. In functional assays, DR8 and IR0 elements act
as independent RAREs, while DR0 does not. Our results reveal an unexpected
diversity in the spacing and topology of binding elements for the RAR-RXR
heterodimer. The differential ability of RAR-RXR bound to DR0 compared to DR2,
DR5 and DR8 to mediate RA-dependent transcriptional activation indicates that
half site spacing allosterically regulates RAR function.
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