MOLECULAR-CLONING, SEQUENCE-ANALYSIS AND EXPRESSION OF PUTATIVE CHICKEN INSULIN-LIKE GROWTH FACTOR-I CDNAS

A cDNA library was constructed from mRNA isolated from the liver of a 5-week-old female broiler chicken; at this age the level of insulin-like growth factor-I (IGF-I) mRNA was expected to be high. Three clones, of sizes 2.3, 0.86 and 0.2 kb, were isolated by using a homologous human (IGF-I) probe. The DNA sequence of these clones has been determined and the potential amino acid sequence deduced. The sequence of the mature chicken IGF-I peptide shows a high degree of homology with IGF-I from other species, providing evidence for the identity of these clones. Alternative splicing of the chicken IGF-I mRNA has been found in the region potentially encoding the leader peptide. This may give rise to two forms of prepeptide, differing in the length and nature of their leader peptide. The 0.86 kb cDNA has been used as a probe to Northern blots of chicken mRNA. A major band of approximately 0.65-0.85 kb was seen, plus several minor bands of larger molecular weights. Analysis of genomic Southern blots shows that there is one copy of the chicken IGF-I gene.