The presence and active transcription of three independent leader exons in the mouse insulin-like growth factor II gene.

The presence of multiple leader exons is one of the common features in the insulin-like growth factor II (IGFII) genes. Among them the 5′ most exon sequence, rE1, was so far reported to be present only in the rat genome. We have found a rE1-homologous sequence in the mouse genome (mE1) and have isolated it by genomic cloning. The mE1 sequence was located in the 5′ region of the IGFII gene and was considered to take an integral part in the mouse IGFII gene construction, just like in the rat gene. Overall homology between mE1 and rE1 regions was approx. 95%. The mE1 was actively transcribed in the newborn tissues and generated approx. 3.8 kb RNA species. Since the other two leader exon sequences were also active, producing 4.6 kb and 3.6 kb RNA species, respectively, transcription units of the mouse IGFII gene were, thus, composed of three leader exon systems.