Complete nucleotide sequence of the chum salmon insulin-like growth factor II gene

Insulin-like growth factor II (IGF-II) is thought to play an important role in development and growth of vertebrates. In contrast to mammals, the IGF-II gene is expressed at a high level from the early stages of embryonic development until the adult stage and IGF-II peptide is produced in virtually all organs of bony fish, indicating a physiological importance of IGF-II 'under water'. Therefore, we describe here the complete nucleotide sequence (accession no. X97225) and organization of the chum salmon IGF-II gene. In addition, the phylogenetic relationship of the IGF-II hormones is analysed. Although the chum salmon contains two non-allelic insulin and IGF-I genes, only one IGF-II gene could be identified. The chum salmon IGF-II gene consists of four exons and is comprised of 7992 bp from the putative transcription initiation site to the poly(A) site. Activation of the only promoter of the salmon IGF-II gene gives rise to a single 4 kb transcript. The fish IGF-II gene is much smaller and simpler organized than its known mammalian counterparts that are governed by several tissue-specific and developmental stage-dependent promoters. All known mammalian IGF-II genes to date have been found to form a conserved linkage group with the insulin and tyrosine hydroxylase (TH) genes and are organized as TH-insulin-IGF-II genomic locus. However, in our study we could find no linkage between the insulin and IGF-II genes, or between the insulin, TH and IGF-II genes, at least within approximately 20 kb of the chum salmon IGF-II genomic sequence. In spite of minor differences, the overall organization of the IGF-II genes turned out to be very similar in bony fish. A limited analysis of the phylogenetic relationship between IGF-II prohormones indeed showed a very conservative phylogenesis of IGF-II in bony fish that may indicate the particular significance of IGF-II in these animals.