Dynamic aspects in the expression of the goat insulin-like growth factor-I (IGF-I) gene: diversity in transcription and post-transcription.

Goat IGF-I (gIGF-I) cDNA was cloned using the reverse transcriptase-polymerase chain reaction (RT-PCR). The cDNA, which was homologous to rat class 1 IGF-I cDNA, was 969 bp long. From the open reading frame found in it, we predicted a 154 amino acid protein consisting of a 49 amino acid signal peptide, a 70 amino acid mature IGF-I peptide, and a 35 amino acid E domain (the COOH-terminal peptide). From the gIGF-I gene, we isolated and sequenced some segments containing four exons that encompassed the entire gIGF-I cDNA sequence. Goat liver RNA was analyzed by RT-PCR, and the nucleotides of the RT-PCR products were sequenced and checked with the nucleotide sequences of the segments from the gIGF-I gene. The gene had three leader exons (1W, 1, and 2, from upstream to downstream) and was transcribed into three kinds of mRNAs (classes 1W, 1, and 2). Another RNA species was detected by RT-PCR analysis of exon 1W. We sequenced it and found that in this transcript, the 3'-portion of exon 1 was inserted between exons 1W and 3, resulting in class 1W-1 del. mRNA. That is to say, the gIGF-I gene had three leader exons and four kinds of mature mRNA.