A NOVEL SEA-URCHIN NUCLEAR RECEPTOR ENCODED BY ALTERNATIVELY SPLICED MATERNAL RNAS

Screening of a genomic library from the sea urchin Strongylocentrotus purpuratus with a human COUP-TF I cDNA probe revealed the presence of a novel gene member of the steroid-thyroid-retinoic acid receptor supe rfamily, which was named SpSHR2 (S. purpuratus Steroid Hormone Recepto r 2). Sequence analysis of the isolated genomic clone revealed that th e DNA binding domain of this orphan receptor is most homologous to the human TR2 receptor. Using this sea urchin genomic fragment as probe, a S. purpuratus embryonic cDNA library was screened and two distinct b ut homologous cDNA clones were isolated. The two cDNAs encode the same DNA binding domain as the SpSHR2 gene and carry an almost identical 3 '-untranslated sequence. One of the clones, however, is missing an ent ire region of about 1100 nt which includes the putative ligand binding domain. Genomic DNA hybridization suggests that SpSHR2 is a single-co py gene in the S. purpuratus genome. Exon skipping during splicing of a single primary transcript appears to be the reason for the different ly sized mRNAs. RNA blot hybridization results suggest that SpSHR2 tra nscripts are stored as maternal RNA in the egg and are not detected be yond the blastula stage. In vitro transcription and translation of the full-length cDNA produced a polypeptide which specifically binds to t he hormone response element in the 5'-flanking region of the sea urchi n actin CyIIIb gene. In vivo labeling of proteins synthesized by cleav age stage embryos followed by immune precipitation of the SpSHR2 prote in using specific antibodies reveals that the maternal SpSHR2 mRNA is being translated during early embryonic development. (C) 1996 Academic Press, Inc.