Identification of antisense transcripts of the chicken insulin-like growth factor-II gene.
Insulin-like growth factor-II (IGF-II) is a polypeptide mitogen which is believed to play an important role in fetal development. The human and rat IGF-II genes are complex transcription units, which contain multiple promoters and polyadenylation sites and which exhibit alternate splicing of their primary transcripts. In order to study IGF-II gene expression during chick embryonic development, we screened a 10-day chick embryo cDNA library with a human IGF-II cDNA probe. We isolated a clone, designated cigf, that was comprised, in part, of sequences homologous to the second coding exon of the human, mouse and rat IGF-II genes. Comparison of the nucleotide sequence of cigf with that of the corresponding genomic clone indicated that cigf was derived from a spliced antisense transcript of the chicken IGF-II gene, which overlapped the second coding exon. Northern blotting experiments with single-stranded RNA probes synthesized using cigf DNA as a template showed that stage 22 and stage 36 chick embryos contained sense strand IGF-II transcripts of 1.4, 2.2, 4.7 and 7.0 kb and antisense strand transcripts of 0.7, 1.3, 1.8, 2.5, 4.9, 6.0 and 8.0 kb. The pattern of sense strand IGF-II transcripts was similar to that previously found in rat fetal tissues. Whilst there are precedents for the transcription of both strands of a single gene, this is the first evidence for antisense transcription of an IGF gene. The functions of the antisense transcripts remain to be determined. These findings demonstrate a further level of complexity in the IGF-II transcription unit and indicate that studies of IGF-II transcript distribution performed with double-stranded probes should be interpreted with caution. They also suggest explanations for the recent finding that IGF-II peptides are present at much lower levels in embryos than expected from the high levels of IGF-II transcripts present.
Taylor
E R
ER
Department of Biochemistry and Molecular Biology, University College, Middlesex School of Medicine, London.
Seleiro
E A
EA
Brickell
P M
PM
D000818:Animals; D001483:Base Sequence; D015152:Blotting, Northern; D015139:Blotting, Southern; D002642:Chick Embryo; D004247:DNA / Q000032:analysis / Q000235:genetics*; D005091:Exons / Q000235:genetics; D006801:Humans; D007335:Insulin-Like Growth Factor II / Q000235:genetics* / Q000378:metabolism; D008969:Molecular Sequence Data; D009693:Nucleic Acid Hybridization; D016372:RNA, Antisense / Q000235:genetics*; D051381:Rats; D014158:Transcription, Genetic
D016372:RNA, Antisense; D007335:Insulin-Like Growth Factor II; D004247:DNA