STEROID THYROID-HORMONE RECEPTOR GENES IN CAENORHABDITIS-ELEGANS

The large family of steroid/thyroid hormone receptor (STR) genes has b een extensively studied in vertebrates and insects but little informat ion is available on it in more primitive organisms. All members posses s a DNA binding domain of zinc fingers of the C2, C2 type. We have use d the polymerase chain reaction with degenerate oligonucleotide primer s covering this region to clone three distinct members of this family from the nematode Caenorhabditis elegans. All three belong to the reti noic acid receptor (RAR), thyroid hormone receptor subfamily of genes. The cDNA of one of these clones shows such a high homology to DHR3, a n early ecdysone response gene found in Drosophila, and MHR3, identifi ed in Manduca sexta, that we have termed it CHR3. Furthermore, the C-t erminal portion of the deduced protein sequence shows a box containing eight identical amino acids among CHR3, DHR3, and MHR3 suggesting an identical specific ligand for these proteins. CNR8 shows homology to N Ak1, and CNR14 has homology to both the RAR-gamma 1 gene and to anothe r ecdysone response gene, E78A. Neither of the latter two cDNAs is a c lear homologue of any known gene and each is distinctive. All of these genes are expressed varyingly in both larval and adult stages of nema tode development as shown by Northern blot analyses. These data demons trate that the STR family of genes is represented in a nematode whose ancestor appeared well before the branching that gave rise to the Arth ropoda and Chordata.