Two conserved essential motifs of the murine immunoglobulin λ enhancers bind B-cell-specific factors

Two highly homologous enhancers associated with the two murine immunoglobulin λ constant-region clusters were recently identified. In order to better understand the molecular basis for the developmental stage-and cell-type-restricted expression of λ genes, we have undertaken an analysis of the putative regulatory domains of these enhancers. By using a combination of DNase I footprinting, electrophoretic mobility shift assay, and site-specific mutations, four candidate protein binding sites have been identified at analogous positions in both enhancers. A mutation of any of these sites decreases enhancer activity. Two of the sites, λA and λB, are essential for enhancer function, and both of these sites appear to bind both B-cell-specific and general factors. Nevertheless, isolated λA and λB sites show no evidence of inherent transactivating potential, alone or together, even when present in up to three copies. We suggest that the generation of transactivating signals from these enhancers may require the complex interaction of multiple B-cell-specific and nonspecific DNA-binding factors.