The nucleotide sequences of the constant portions of mouse immunoglobulin gamma 1 and gamma 2b genes were compared. A remarkable homology was found in a long (about 500 nucleotides) continuous segment including the entire CH1 coding region and about the first half of the first intervening sequence. Furthermore, comparison of amino acid sequences of four gamma-class chains revealed that the CH1 domain shows limited divergence among gamma 1, gamma 2a, and gamma 2b. Interestingly, the homology region extends to the CH2 domain in gamma 2a and gamma 2b. These findings suggest that, during their evolution, a double unequal crossing-over event has taken place at different intervening sequences, resulting in the transfer of the DNA segment coding for the CH1 domain or CH1-CH2 domains. A possible evolutionary implication for such an "intervening sequence-mediated domain transfer" event is discussed.
Amino Acid Sequence Animals