Identification of multiple B-cell transcriptional repressor elements in S mu-C mu intron of mouse IgH chain locus

The S mu-C mu intron of the IgH chain locus is conserved in rodents, but its biological function is unknown. It has been shown that switch recombination breakpoints are concentrated within the repetitive sequences in the S mu region in mitogen-activated normal B cells. In Ig-secreting hybridomas these breakpoints occur most frequently at the most 5' end, and immediately upstream of the S mu DNA. The S mu-C mu intron appears remarkably protected from recombination. Because the nucleoprotein complexes that drive transcription and recombination may overlap, the transcriptional characteristics of this fragment were studied. The cis-acting regulatory elements in the S mu-C mu intron were identified by ligating the entire intron, or a series of subfragments to the TK promoter and bacterial chloramphenicol acetyltransferase gene. Expression of these constructs was tested in activated B cells and the nonlymphoid cell lines HeLa and HepG2. The complete S mu-C mu intron (1 kb) had a negative effect on TK promoter activity in activated B cells only when placed upstream of the promoter, in both orientations. Segmentation of the S mu-C mu intron has revealed that this region contains multiple negative elements active in B cells. A subfragment located at the 3' end of the S mu-C mu intron contains a B-cell-specific negative element, while the subfragment located at the 5'end has cell-type-independent repressing activity.