Evolution of Insulin-Like Growth Factor II: Characterization of the Mouse IGF-II Gene and Identification of Two Pseudo-Exons

Abstract

We have cloned the mouse insulin-like growth factor II (IGF-II) gene as a series of overlapping cosmid and λ recombinants and have characterized its six exons. The gene extends over approximately 12 kb of mouse chromosome 7 and is located 18 kb 3′ to the insulin 2 gene and in the same transcriptional polarity. Exons 1–3 encode distinct 5′ untranslated regions and are transcribed by three different promoters, P₁, P₂, and P₃, into three IGF-II mRNAs sharing common coding and 3′ untranslated sequences. Promoters P₂ and P₃ each contain a TATA box and appear to direct transcription from single initiation sites. By contrast, exon 1 has three major transcriptional start sites distributed over 556 nucleotides, and P₁ lacks a TATA region and other typical transcriptional control sequences. Exons 4–6 code for the 180-amino-acid IGF-II precursor, and exon 6 also contains a 3,045-nucleotide 3′ untranslated region which ends at a single polyadenylation site. In addition to six functional IGF-II exons, we identified two 5′ "pseudo-exons," which appear to be evolutionarily retained remnants of an alternative promoter-exon cassette that is active in human IGF-II. Loss of the homolog of this promoter, which directs "adult-specific" expression of the IGF-II gene in some human tissues, may explain the disappearance of this growth factor from most murine tissues in the early postnatal period.