Relocalization of retinoic acid receptors from non-canonical to canonical spaced binding elements during embryoid body differentiation

Retinoic Acid Receptors (RARs) bind RA-response elements in regulatory regions of their target genes. While canonical RAREs comprise direct repeats of the consensus 5’-RGKTCA-3’ sequence separated by 1, 2 or 5 nucleotides (DR1, DR2, DR5), we show that shortly after RA treatement of mouse embryoid bodies or F9 cells, RARs occupy a large repertoire of DR0, DR2, DR5, DR8 and IR0 elements. In vitro, RAR-RXR bind these non-canonical spacings 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 and acts as an RARE. At later stages of embryoid body differentiation, RARs relocalise to a restricted repertoire of sites comprising predominantly DR5 elements. Differentiation thus involves genomic relocalisation of RARs, and a switch from DR0 and DR8 at early times to DR5 at later stages. Overall design: Examination of genomic localisation of RAR in differentiating embryoid bodies.