Tissue‐specific deoxyribonuclease I‐hypersensitive sites in the vicinity of the immunoglobulin Cλ cluster of man

During B cell development, the onset of DNA rearrangements, expression, and somatic hypermutation of Ig genes are regulated through the complex interaction of cis‐acting elements with trans‐acting factors. Our aim is to identify DNA elements required during activation of the human Igδ light chain genes. Determination of deoxyribonuclease (DNase) I‐hypersensitive sites in complex regulated genes can lead to the identification of sequence elements which would have been overlooked by employing transient transfection protocols. We have therefore investigated the chromatin structure of human J‐Cλ genes and identified three DNase I‐hypersensitive sites (HSS‐1, −2, and −3) within an 8‐kb region downstream of the J‐Cλ7 gene. HSS‐2 and HSS‐3 are B cell specific. The DNase I‐hypersensitive sites are also present in X‐producing cell lines which have not rearranged the Igδ locus and produce germ‐line J‐Cλ transcripts. We conclude that in mature B cells, both x and λ loci are in an active structure regardless of the type of light chain they produce. This suggests that the chromatin structure of both loci is opened early in B cell development and that the active structure persists in mature B cells. The observed temporal order (first x, then δ) of activation can be explained by consecutive synthesis of the appropriate regulating factors and the tight regulation of the recombination machinery through the products of L chain gene rearrangements.