The recombination process that joins a VK to a JK segment of an immunoglobulin gene generates a second, reciprocal recombination product called f fragment. In this second product the regions flanking the VK and JK 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 JK1 flanks; the VK derived moiety of f Daudi and f41 could be traced back to known germline VK 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 VK-JK recombination products, the f fragments were found to lack somatic mutations. The structures of the f fragments are discussed in the context of the VK-JK rearrangement mechanism.
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