A homolog of retinoid X receptors (RXR), named PmRXR, was cloned from the b
udding ascidian, Polyandrocarpa misakiensis. Gel-shift assays revealed that
PmRXR and a previously identified P. misakiensis retinoic acid receptor (P
mRAR) formed a complex to bind vertebrate-type retinoic acid response eleme
nt (RARE). Transfection assays were carried out using a reporter gene conta
ining a RARE upstream of lacZ. Two chimeric effector genes were constructed
by placing PmRXR and PmRAR cDNA fragments (containing the DNA-binding, lig
and-binding and ligand-dependent transactivation domains) downstream of the
human RXR alpha and RAR alpha cDNA (covering the N-terminal coding region)
, respectively. Each chimeric cDNA was ligated to a notochord-specific enha
ncer. In case the embryos were transfected with all three transgenes and tr
eated with retinoic acid (RA), the reporter gene was activated in the notoc
hord cells. The result suggests that the PmRXR/PmRAR complex functions as a
n RA-dependent transcriptional activator. The PmRXR mRNA was detected in a
mesenchymal cell type, called glomerulocyte, in the developing Polyandrocar
pa bud. As this cell type has been shown to express PmRAR mRNA, it seems po
ssible that the PmRXR/PmRAR complex mediates RA signaling in this cell type
to induce the expression of genes involved in the morphogenesis of the dev
eloping bud.