Structure of the chicken interferon-gamma gene, and comparison to mammalian homologues.
The sequence of the chicken interferon-gamma (ifn-gamma) gene was determined, one of the first non-mammalian cytokine gene structures to be elucidated. Initial genomic clones were amplified from chicken genomic DNA and were used to isolate a cosmid clone covering the entire gene for sequencing. The exon:intron structure of chicken ifn-gamma is very similar to those of its mammalian homologues, with the exception of the third intron, which is markedly shorter in the chicken. The first exon contains both 5' UTR and signal sequence and the first 22 aa of the mature protein. The remainder of the coding region lies in exons 2-4. Exon 4 also encodes the stop codon and the 3' UTR, including two possible polyadenylation signals. A number of potential regulatory sequences similar to those found in mammals have been identified, in the promoter, in each intron and in the 3' UTR. In the promoter, these include the TATAATA- and CCAT-boxes, a consensus GATA motif in the reverse orientation and a potential NF-kappa B binding site. Other regulatory elements identified in the promoters of mammalian ifn-gamma genes are absent. Internal to the gene structure, regulatory sequences identified include elements found in the DNase I hypersensitivity region of the first intron of the human ifn-gamma gene and several potential NF-kappa B binding sites. The 3' UTR contains an AT-rich sequence, including nine repeats of the 'instability' motif ATTTA. As in mammals, chicken ifn-gamma is a single copy gene. The gene is highly conserved, with no polymorphisms yet identified using either RFLP or SSCP in the coding region. However, promoter sequence polymorphisms between different inbred lines of chickens have been identified, with possible links to disease resistance.