DUPLICATE INSULIN-LIKE GROWTH FACTOR-I GENES IN SALMON DISPLAY ALTERNATIVE SPLICING PATHWAYS

Insulin-like growth factor (IGF-I) is a single chain, 70-amino acid, peptide hormone, which is essential to the process of growth and differentiation in higher vertebrates. IGF-I exhibits a very high degree of conservation throughout vertebrate evolution, and recent studies in fish have shown improved rates of growth in salmonids treated with exogenous recombinant mammalian IGF-I, implying a role for this hormone in fish similar to that in mammals. This paper reports the identification and sequencing of four different liver-derived cDNAs encoding IGF-I from chinook salmon, Oncorhynchus tshawytscha. The different cDNAs all contain sequences that encode an identical mature IGF-I protein. The differences between the four cDNAs are of two types: 1) single basepair changes and 2) additional blocks of sequence occurring at one site within the E-domain regions of three of the four cDNAs. The sequence data indicate that each of the cDNAs identified was transcribed from a different IGF-I gene, and analysis of the chinook salmon genome by Southern blotting supports the existence of four distinct IGF-I genes and suggests that there are at least two IGF-I loci in salmon. Although, analysis of total RNA from salmonid liver identifies only a single major transcript of 4200 nucleotides, polymerase chain reaction analysis suggests the presence of two structurally different salmonid IGF-I genes with the potential for multiple splicing alternatives of the IGF-I transcript.