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From the Departments of Medicine and Molecular Biology & Genetics, The Johns Hopkins University
School of Medicine, Baltimore Maryland 21205
The process of V(D)J recombination is crucial for regulating the development of B cells and for
determining their eventual antigen specificity. Here we assess the developmental regulation of
the V(D)J recombinase directly, by monitoring the double-stranded DNA breaks produced in
the process of V(D)J recombination. This analysis provides a measure of recombinase activity at
immunoglobulin heavy and light chain loci across defined developmental stages spanning the
process of B cell development. We find that expression of a complete immunoglobulin heavy
chain protein is accompanied by a drastic change in the targeting of V(D)J recombinase activity, from being predominantly active at the heavy chain locus in pro-B cells to being exclusively restricted to the light chain loci in pre-B cells. This switch in locus-specific recombinase
activity results in allelic exclusion at the immunoglobulin heavy chain locus. Allelic exclusion is
maintained by a different mechanism at the light chain locus. We find that immature, but not
mature, B cells that already express a functional light chain protein can undergo continued light
chain gene rearrangement, by replacement of the original rearrangement on the same allele. Finally, we find that the developmentally regulated targeting of V(D)J recombination is unaffected by enforced rapid transit through the cell cycle induced by an Eµ-myc transgene.
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