MULTIPLE RETINOID-RESPONSIVE RECEPTORS IN A SINGLE CELL - FAMILIES OF RETINOID X RECEPTORS AND RETINOIC ACID RECEPTORS IN THE XENOPUS EGG

In a search for nuclear hormone receptors expressed in early development we found that Xenopus laevis eggs contain mRNAs from two retinoic acid receptor genes (xRAR-alpha and xRAR-gamma) and two retinoid "X" receptor genes (xRXR-alpha and xRXR-gamma). We also show that RXRs are members of a family of at least three genes, thus expanding the number of genes encoding retinoic acid-responsive transcription factors to six. With the exception of xRXR-gamma, these maternal mRNAs are degraded before gastrulation. The RXRs isolated are differentially activated by retinoic acid and by 3,4-didehydroretinoic acid. Considered together, these four receptors provide a molecular basis for the pleiotropic effects of retinoic acid on early development, and their pattern of expression suggests a role for retinoic acid at the earliest stages of embryonic determination.