Abelson Murine Leukemia Virus-transformed

Transformation of murine adult bone marrow cells by Abelson Murine Leukemia Virus (A-MuLV)' can result in the generation of permanent cell lines (Abelson lines) representing early stages of B lymphocyte differentiation (1) . Because it is difficult to obtain large numbers of B cell precursors capable of Ig gene recombination from normal cell populations, Abelson cell lines provide an accessible model system in which to study the development and regulation of immunoglobulin (Ig) gene rearrangement. Many A-MuLV transformants exhibit characteristics of the pre-B cell stage of B cell development, i.e ., rearranged and expressed heavy (H), but not light (L), chain genes (2-6). Active Ig H-chain gene rearrangement has been found to occur in certain A-MuLV-transformed pre-B cell lines during maintenance in tissue culture (3). In addition, some Abelson lines that have rearranged H-chain genes proceed to recombine K light chain genes (7-10) in culture, and some have been observed to spontaneously undergo deletion of K constant region (CK) genes (7, and this report). We have previously described a class of recombination events that are associated with Crc deletion (11, 12). These recombinations involve a segment of DNA, designated as RS (for recombining sequence), which joins directly to either a K variable region (VK) gene segment or to the JK-Ca intron (11, 12). RS DNA lies at least 15 kb downstream of the Ca exon on chromosome 6; thus either type of RS recombination results in CK deletion (12) . RS recombination events are mediated by recognition sequences that are similar to those involved in V-J joining (12) . RS recombinations are found frequently in hybridomas producing X light chains, but not in x-producing hybridomas (11), suggesting RS recombination may be linked to X gene rearrangement in developing B cells . In addition, a recombining DNA element (kde) that appears to be analogous to RS and that is involved with Ca deletion in human B cells has also been identified (13) . RS DNA and kde segments share extensive nucleotide sequence homology, indicating that these elements have been evolutionarily conserved (13a). These results