Identification of an octamer-binding site in the mouse kappa light-chain immunoglobulin enhancer

A 215-base-pair (bp) region of the mouse MOPC 41 kappa light-chain immunoglobulin gene enhancer has been analyzed for specific binding of lymphoid and nonlymphoid nuclear factors. Mobility shift assays with a series of overlapping DNA fragments have mapped DNA-binding sites for three unique factors. The B-cell-specific (OTF-2) and ubiquitous (OTF-1) octamer-binding transcription factors specifically bound to a site centered about 136 bp 5' of the nuclear factor NF-kappa B site. A third specific factor, NF-kappa E, bound to a site that was about 75 bp 5' of the NF-kappa B site and within a region important for enhancer function. This novel factor was found in both mature B and HeLa cell nuclei. B-cell OTF-2, B-cell OTF-1, and HeLa OTF-1 bound to the kappa enhancer and kappa promoter octamer sites with similar affinities despite a 2-bp difference in the kappa enhancer octamer sequence. However, DNase I footprint analyses indicated that affinity-purified OTF-2 bound both to the enhancer OTF site and, surprisingly, to 80 bp of A + T-rich flanking sequence. Moreover, methylation interference studies demonstrated distinct differences in OTF interactions between the consensus octamer in the kappa promoter and the nonconsensus octamer identified in the enhancer. This novel observation of an OTF-binding site in the kappa enhancer provides a common link with the OTF sites in the promoter-proximal regions of all kappa promoters and thus mirrors the structural arrangement of OTF sites found in the promoters and enhancers of immunoglobulin heavy-chain genes.