Two Forms of Loops Generate the Chromatin Conformation of the Immunoglobulin Heavy-Chain Gene Locus

The immunoglobulin heavy-chain (IgH) gene locus undergoes radial repositioning within the nucleus and locus contraction in preparation for gene recombination. We demonstrate that IgH locus conformation involves two levels of chromosomal compaction. At the first level, the locus folds into several multi-looped domains. One such domain at the 30 end of the locus requires an enhancer, Em; two other domains at the 50 end are Em independent. At the second level, these domains are brought into spatial proximity by Em-dependent interactions with specific sites within the V-H region. Em is also required for radial repositioning of IgH alleles, indicating its essential role in large-scale chromosomal movements in developing lymphocytes. Our observations provide a comprehensive view of the conformation of IgH alleles in pro-B cells and the mechanisms by which it is established.