In mammals and the amphibian, Xenopus, isotypes of antibodies have been shown to be changed through class switch recombination within the IgH chain gene locus. Here, we identified switch (S) repetitive sequences in the 5' introns of the Ig C(μ) and C(γ) genes of the chicken. The S(μ) region is composed of two homologous regions, S(μ)1 and S(μ)2. The S(μ)1 region is an upstream 3.7 kb sequence composed of 37 repeats of a consensus sequence containing tandem repeats of the decamer ACCAGTATGG. The S(μ)2 region is a downstream 1.4 kb sequence consisting of simple tandem repeats of a decamer CCCAGTACAG. The S(γ) region contains repeats of the decamer TATGGGGCAG. Analysis of chicken IgG-producing hybridomas revealed that the C(μ) gene was deleted from the chromosome by the recombination occurring between the S(μ) and S(γ) regions. Recombination breakpoints at the C(μ) gene of splenocytes from an immunized chicken were scattered around the S(μ) region and two such breakpoints, the precise position of which were determined, were located within possible hairpin loop structures at the palindromic sequence of S(μ)1. A primordial palindromic sequence from which the prevalent switch repeat motifs of mammals, chickens and amphibians may have diverged is presented.
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