Duck (Anas platyrhynchos), Japanese quail (Coturnix coturnix japonica) and other avian interleukin-2 reveals significant conservation of gene organization, promoter elements and functional residues.

We compared the gene, promoter and cDNA sequences, and the predicted protein structure of duck and quail interleukin-2 (IL-2), a major immunomodulatory cytokine, with the known sequences of other avian and human IL-2. Analysis of the gene organization showed significant similarity with the overall organization of mammalian IL-2 genes, with four exons and three introns and a very short 5' untranslated regions. The second intron was the biggest in all the IL-2 sequences. The third intron was of similar size in chicken and quail, whereas in duck it was found to be slightly longer. Promoter sequence analysis of the IL-2 gene revealed remarkable conservation of the functionally important residues. The transcription factor binding sites such as those for AP-1, NF-AT, CD 28 RE and OCT, the TATA box and the predicted transcription start site with respect to chicken IL-2 sequence showed total conservation in duck, quail and turkey IL-2 promoters. Comparative analysis of the avian IL-2 cDNAs such as those of chicken, turkey, duck, quail, goose and Muscovy duck, revealed significant conservation of the nucleotide and predicted amino acid sequences. They showed nucleotide identity levels varying from 75% to 85%, amino acid level identity from 58% to 72% and amino acid similarity from 71% to 80% with each other. In the predicted protein secondary structure, the four essential alpha-helices and the hydrophobic amino acids in the heptad repeats forming the core structure of IL-2 molecules were conserved in all the avian and the human IL-2. The present study reveals high-level conservation of the gene; cDNA structure and regulatory elements of avian IL-2. This indicates highly conserved functions and probable functional cross-reactivity of this major immunomodulatory cytokine among birds.