The ovine insulin-like growth factor-I gene: characterization, expression and identification of a putative promoter

Genomic DNA encoding the ovine insulin-like growth factor-I (IGF-I) gene was cloned and sequenced. The predicted amino acid sequence of the mature form of ovine IGF-I was highly homologous to that of human, rat and mouse. Analysis of the DNA sequence between exons 1 and 2 suggested the existence of an alternative 5' exon (exon 1A) and this was confirmed by polymerase chain reaction (PCR) analysis of sheep liver mRNA. Primer extension of mRNA from exon 1A indicated a class of transcripts which initiated at a point 32 nucleotides 5' to the Met codon of exon 1A to give a mRNA comprising exons 1A, 2, 3 and 5. In liver these transcripts co-existed with the alternative exon 1, 2, 3 and 5 mRNA form. Analysis by PCR of the 3' terminus of liver RNA indicated heterogeneity arising from multiple polyadenylation sites; however, of the two possible alternatively spliced 3' exons, only exon 5 could be detected. Expression of IGF-I mRNA, as measured by a solution hybridization/RNase protection assay, predominated in the liver of the neonate and the late-gestation fetus; however, lower levels of expression were seen in multiple tissues throughout fetal and neonatal development.