Organisation and Promoter Activity of the Retinoic‐acid‐induced‐heparin‐binding (RIHB) Gene

Abstract

The heparin-binding protein, RIHB, is encoded by a gene that is expressed temporarily during the early period of chicken embryogenesis. We have now isolated genornic clones of the chicken RIHB gene, determined its transcription initiation sites, characterized its promoter region and established the functional activity of the RIHB gene promoter. The 5' flanking region and the beginning of the first exon revealed a structural organisation characteristic of housekeeping and growth-control-related genes. It lacked canonical TATA or CAAT boxes but contained several GC boxes. The structure of the RIHB gene is compared with those of the human and mouse midgestation kidney (MK) genes. In the coding exons, the similarities between the three genes are striking. In contrast, the structure and location of the first non-coding exon is different. Analysis of the promoter activity suggests an indirect regulation of transcription by retinoic acid and demonstrates a high degree of complexity of the regulation of the RIHB gene.We have previously purified to homogeneity a secreted heparin-binding protein from the chicken embryo [l]. A cDNA clone was isolated from a chicken library [2] and the deduced sequence predicts that the retinoic-acid-inducedheparin-binding (RIHB) gene contains 121 amino acids and is very rich in basic amino acids and in cysteine residues. Northern-blot analysis of RNA extracted from whole embryos indicates that the level of RNA expression is high during early embryonic development (day 2), then decreases and is barely detectable by this method after day 15 [2]. The secreted protein is present in basement membranes and around many types of cells, apparently irrespective of their embryonic origins, during early embryogenesis. The RIHB mRNA is expressed in differentiating chondrocytes and the protein is accumulated in the hypertrophic chondrocytes [3], suggesting a role in cartilage differentiation. In vitro, the synthesis of this protein is controlled by retinoic acid in primary cultures of embryonic chicken chondrocytes [4] and myoblasts [5].RIHB belongs to a new heparin-binding protein family of which the principal characteristics are the ability to bind heparin, a molecular mass of approximately 14-15 kDa, basicity, the conservation of cysteine and tryptophan residues in the amino acid sequence, a neurotrophic action and an induction by retinoic acid in some in vitro models. Members of this family include the product of the midgestation kidney (MK) gene characterised in the mouse [6] Abbreviations. RIHB, retinoic-acid-induced heparin-binding; MK, midgestation kidney; HBGAM, heparin-binding growth-associated molecule; RA, retinoic acid.Note. The sequence reported in this paper has deposited with the EMBL data bank and is available under accession number X 76482.(HBGAM), characterised in the rat,mouse, cow, human and in the chicken. This second protein has been assigned several names, reflecting the diverse laboratories in which it was identified (listed in [S]). Sequence similarities and a similar expressi...