DYAD SYMMETRY WITHIN THE MOUSE 3'-IGH REGULATORY REGION INCLUDES 2 VIRTUALLY IDENTICAL ENHANCERS (C-ALPHA-3'E AND HS3)

The transcription of the murine Ig heavy chain locus is regulated not only by the intronic enhancer, E mu, but also by a 3' regulatory regio n located downstream of the C alpha membrane exon. Several DNase I-hyp ersensitive sites (hs1-4) and enhancer elements (e.g., C alpha 3'E) ha ve been identified in this 3' regulatory region, and some of these wer e suggested to comprise a locus control region. However, little is kno wn about the coordinate regulation or function of these individual ele ments, Here we provide evidence that C alpha 3'E and hs3 are virtually mirror images of each other and demarcate the edges of an similar to 25-kb region of quasi-dyad symmetry with 3'alpha E(hs1,2) at its cente r. Flanking 3'alpha E(hs1,2) are inverted repeats and families of repe titive sequences uniquely located in this region. We have observed tha t, like S'alpha E(hs1,2) and hs3, C alpha 3'E is DNase I hypersensitiv e in plasma cell lines, but not in a pre-B cell line. Additionally, we found that C alpha 3'E and hs3 show significant transcriptional syner gy in transfection assays only in a plasma cell line. The DNA topology of the 3' regulatory region coupled with new and existing data on the activity of its individual enhancers during B cell differentiation le ad us to propose a biphasic model for the activity of this region. Acc ording to our model, one unit, consisting of the 3'-most enhancer, hs4 , is active early and throughout B cell development. The second unit, which comprises C alpha 3'E, 3'alpha E(hs1,2), and hs3, becomes active later in development, when it contributes to such processes as class switching and increased levels of Ig heavy chain gene transcription in plasma cells.