Characterization of multiple transcription initiation sites of the ovine insulin-like growth factor-I gene and expression profiles of three alternatively spliced transcripts.

Alternative splicing of ovine insulin-like growth factor-I (IGF-I) transcripts generates three different mRNAs. Class 1 and class 2 transcripts contain exons 1 and 2 spliced to exon 3, respectively. A novel IGF-I mRNA containing exon W is spliced to exon 3 and has been located upstream of exon 1. No in-frame methionine codon was present in exon W and therefore translation is proposed to initiate at the methionine codon present in exon 3. Using primer extension, transcription initiation sites were found 179, 336, and 368 nucleotides upstream of exon 1 and 86, 96, 131, and approximately 850 nucleotides upstream of exon 2. The locations of these transcription initiation sites are well conserved among mammalian and avian IGF-I genes. Expression of exon 1-, 2-, and W-specific transcripts was examined in brain, heart, kidney, liver, lung, skeletal muscle, and spleen from adult ewes or 75-day fetal lambs using a reverse transcriptase-polymerase chain reaction assay. Exon 1 transcripts were the most abundant and found in all fetal and adult tissues. Exon 2 transcripts were found in all tissues and were generally expressed the highest in adult liver. Exon W transcripts were also found to be expressed in all tissues examined. Thus, the three alternatively spliced ovine IGF-I transcripts were expressed in a variety of fetal and adult tissues.