Reciprocal recombination products of V_k-J_k joining reactions in human lymphoid cell lines

The recombination process that joins a V_K to a J_K segment of an immunoglobulin gene generates a second, reciprocal recombination product called f fragment. In this second product the regions flanking the V_K and J_K segments in the germline are joined in a head to head fashion. We now analysed f fragments in the human lymphoid cell lines Daudi, JI and IARC/BL41. All three f fragments contain J_K1 flanks; the V_K derived moiety of f Daudi and f41 could be traced back to known germline V_K genes. There is a precise head to head joining of the heptanucleotide signal sequences in f Daudi and fJI while in f41 six nucleotides are present between the signal sequences. In contrast to the V_K-J_K recombination products, the f fragments were found to lack somatic mutations. The structures of the f fragments are discussed in the context of the V_K-J_K rearrangement mechanism.