The aim of this project is to determine the earliest phylogenetic occurrence of genes of the Thyroid/Steroid hormone receptor family in order to better understand their function. More than thirty members of this gene family are known but there are few data on their occurrence in primitive forms. Many of these gene products, although they possess a ligand binding domain have no known ligands. To obtain clones we have used degenerate primers involving the DNA binding domain which is well preserved in this family, and have devised procedures including PCR to allow efficient cloning of the relevant genes. We have previously isolated a sequence for medusa but have so far been unable to get a full length cDNA. We isolated three clones from ciona intestinalis and Dr. Janinni in Italy is currently working on one of them. We have now isolated and completely sequenced three cDNAs from C elegans (a primitive round worm). One of these clones (ce3) shows highest amino acid homology to DHR 3, one of the ecdysone response genes in drosophila. Another, ce8 shows lesser homology to an early human response gene(nak1), and a low but significant homology to NOT1, a yeast general negative regulator of transcription. Ce14 is most homologous to the retinoic acid receptor gene. All the homologies outside the DNA binding domain are so low that the genes we describe appear to be new members of this family. Northern analysis shows that ce3 and ce8 are highly expressed in embryos and in larval stages. All three genes are expressed to about the same extent in adults. In preliminary experiments with in situ hybridization using a digoxigenin labelled riboprobe and an antibody conjugated with alkaline phosphatase, results similar to the Norther experiments were obtained.
