Polymorphism in immunoglobulin heavy chains suggesting gene conversion.

Complete heavy (H) chain variable region (V region; amino acids 1-118) sequences have been determined for three phosphocholine (PCho)-binding monoclonal antibodies of CBA mouse strain origin. Two of these were found to differ from the sequence of the BALB/c T15 germline VH segment (segment of the V region that includes amino acids 1-95) at four positions but were identical to the allelic form of T15 (C3) found in C57BL. The third VH segment, HP101.6G6 (6G6), was clearly the product of a second, related VH gene, probably the allele of the BALB/c V11 gene, a second member of the P-Cho VH gene family. Thus, more than one VH gene is capable of encoding heavy chains of PCho-binding antibodies. The 6G6 VH segment differs from VII at seven positions; four of these distinguishing amino acids are encoded in other membranes of the PCho VH gene family. We postulate that the origin of the 6G6 VH sequence can most easily be explained by a process of gene conversion occurring between the least three members of the PCho VH family.