Isolation of a second nonallelic insulin-like growth factor I gene from the salmon genome

We have characterized a second nonallelic insulin-like growth factor-I (IGF-I) gene in the chum salmon (Oncorhynchus keta) genome. This gene, IGF-I.2, differs from the previously described chum salmon IGF-I gene, IGF-I.1, in the E peptide-coding portion of exon 3; specifically, the IGF-I.2 gene lacks one codon present in the IGF-I gene and contains two potential splice donor sites at the 3' end of exon 3 rather than the single, more distal site present in the IGF-I.1 gene. The expression of these two IGF-I genes could give rise to as many as six IGF-I mRNA species, each of which would encode a unique E-peptide moiety of the IGF-I prohormone. Thus, the presence of multiple, distinct IGF genes adds an additional level of complexity to IGF-I gene expression and IGF-I biosynthesis in salmon.