Structural organization of the human S-antigen gene. cDNA, amino acid, intron, exon, promoter, in vitro transcription, retina, and pineal gland.

S-Antigen (S-Ag) is a major soluble photoreceptor protein involved in the visual transduction cascade. Several S-Ag cDNAs and a gene coding for human S-Ag were isolated from cDNA and gene libraries. The gene sequences of the coding, noncoding, and 5'-flanking regions of the gene were determined. The S-Ag gene was approximately 50 kbp (kilobase pairs) in length and contained 16 exons and 15 introns. The length of most exons was less than 100 base pairs (bp) and the smallest one was only 10 bp. In contrast, the length of most introns was larger than 2 kbp, and the gene comprised 97% intron and 3% exon. The splice sites for donor and acceptor were in good agreement with the GT/AG rule. The S-Ag protein of 403 amino acid residues was translated from a mRNA of 1.9 kbp, and the mRNA was transcribed from a gene of 50 kbp. The 5'-flanking region of the gene, approximately 1.1 kbp long, had no known regulatory elements for transcription such as TATA, GC, and CCAAT boxes. Interestingly, the 5'-flanking region had promoter activity in an in vitro transcription assay using a nuclear extract of rat brain. A major transcription start site was found at 387 bp upstream from the translation start site ATG. Our results indicate that the sequence of S-Ag promoter differs from other known promoters and may, perhaps, be specific for photoreceptor rod cells and pinealocytes.