Nucleotide sequence of the promoter region of Sparus aurata insulin-like growth factor I gene and expression of IGF-I in eggs and embryos

A genomic fragment of 3.1 kb, containing the promoter region of Sparus aurata insulin-like growth factor I (IGF-I) gene has been cloned and sequenced. This fragment contains exon 1 and exon 2 of Sparus aurata IGF-I gene. These two exons are identical in organization to the reported chum salmon IGF-I gene. Exon 1 contains 5′ untranslated region and part of the signal peptide. Exon 2 codes for part of the signal peptide and part of the B domain. Nested polymerase chain reaction (PCR) revealed that a fragment was amplified from liver RNA when a common primer in the translated region and a primer located between 375 and 395 nucleotides upstream of the first methionine were used. No such amplification was obtained when the primer was located between 414 and 434 nucleotides upstream of the first methionine, suggesting that the first exon in Sparus IGF-I gene starts 400-110 nucleotides upstream of the first methionine. Expression of IGF-I mRNA was studied in Sparus aurata using reverse transcription (RT)-PCR. An amplified fragment was found in unfertilized eggs and in embryos 4, 8, and 12 hours after fertilization, when oligonucleotides specific for Sparus aurata IGF-I cDNA were used. A similar fragment was found when adult liver or one-day larval RNA were used. This fragment hybridized in a Southern blot to salmon IGF-I cDNA. These results demonstrate that the structure of IGF-I gene has been conserved in teleosts and IGF-I transcripts are present in fish during embryogenesis, probably of maternal origin.