RETINOIC-ACID-X RECEPTOR IN THE DIPLOBLAST, TRIPEDALIA-CYSTOPHORA

Nuclear hormone receptors comprise a characteristic family of transcri ption factors found in vertebrates, insects and nematodes, Here we sho w by cDNA and gene cloning that a Cnidarian, Tripedalia cystophora, po ssesses a retinoid receptor (jRXR) with remarkable homology to vertebr ate retinoic acid X receptors (RXRs), Like vertebrate RXRs, jRXR binds 9-cis retinoic acid (K-d = 4 x 10(-10) M) and binds to the DNA sequen ce, PuGGTCA as a monomer in vitro. jRXR also heterodimerizes with Xeno pus TR beta on a thyroid responsive element of a direct repeat separat ed by 4 bp. A jRXR binding half-site capable of interacting with (His( 6))jRXR fusion protein was identified in the promoters of three T. cys tophora crystallin genes that are expressed highly in the eye lens of this jellyfish. Because crystallin gene expression is regulated by ret ionoid signaling in vertebrates, the jellyfish crystallin genes are ca ndidate in vivo targets for jRXR. Finally, an antibody prepared agains t (Hiss)jRXR showed that full-length jRXR is expressed at all developm ental stages of T, cystophora except the ephydra, where a smaller form replaces is. These data show that Cnidaria, a diploblastic phylum anc estral to the triploblastic invertebrate and subsequent vertebrate lin eages, already have an RXR suggesting that RXR is an early component o f the regulatory mechanisms of metazoa.