Class switch recombination of the chicken IgH chain genes: implications for the primordial switch region repeats

In mammals and the amphibian, Xenopus, isotypes of antibodies have been sho wn to be changed through class switch recombination within the IgH chain ge ne locus, Here, we identified switch (S) repetitive sequences in the 5' int rons of the Ig C-mu and C-gamma genes of the chicken, The S-mu region is co mposed of two homologous regions, S(mu)1 and S(mu)2. The S(mu)1 region is a n upstream 3.7 kb sequence composed of 37 repeats of a consensus sequence c ontaining tandem repeats of the decamer ACCAGTATGG. The S(mu)2 region is a downstream 1.4 kb sequence consisting of simple tandem repeats of a decamer CCCAGTACAG, The S-gamma region contains repeats of the decamer TATGGGGCAG, Analysis of chicken IgG-producing hybridomas revealed that the C-mu gene w as deleted from the chromosome by the recombination occurring between the S -mu and S-gamma regions. Recombination breakpoints at the C-mu gene of sple nocytes from an immunized chicken were scattered around the S-mu 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(mu)1. A primordial palindromic sequence from which the prevalent swit ch repeat motifs of mammals, chickens and amphibians may have diverged is p resented.