Identification of a novel regulatory element within the murine Igκ locus

Using Long PCR, we have physically linked the variable (Vκ) and joining region (Jκ) gene segments of the murine immunoglobulin kappa locus (Igκ). Within this sequence (VκJκIVS) we have identifed two clusters of DNaseI hypersensitive sites (HSS). One cluster of 4HSS contains two pre-B cell specific sites. To determine the function of the 4HSS cluster, it was assayed in luciferase reporter constructs containing a Vκ promoter and the Igκ intronic enhancer. We observe up to 95% silencing of expression in several pre-B cell lines and a loss of silencing activity in the mature B cell lymphoma M12.4. Complete silencing activity requires a 2.3 kb fragment that includes the pre-B cell specific sites. We are currently assaying the Igκ silencer for its effect on rearrangement of episomal substrates. Preliminary evidence suggests that the Igκ silencer also functions as a rearrangement enhancer in an orientation dependent manner. Our results suggest a model for Igκ locus activation in which the Igκ silencer/rearrangement enhancer may act as focal point around which the rearrangement machinery is organized.