Comparison of Mouse and Rabbit Eiκ Enhancers Indicates That Different Elements Within the Enhancer May Mediate Activation of Transcription and Recombination

The intronic Ig kappa-light chain enhancer (Eikappa) has been implicated in regulation of transcription and Vkappa-Jkappa recombination at the kappa locus. To identify sequences within the Eikappa enhancer which are involved in control of recombination, we have made use of the finding that the Eikappa element from the rabbit b9 kappa locus is capable of inducing rearrangement, but not transcription of kappa genes in mouse lymphoid cells. We have therefore compared the binding of murine nuclear proteins to the mouse and rabbit Eikappa elements. DNase I footprinting and gel mobility shift assays indicate that only the kappaB, kappaE1, and kappaE2 sites of the rabbit enhancer are able to interact with murine trans-acting factors. Moreover, although the rabbit kappaB site binds murine NF-kappaB p50/p50 and p50/p65 complexes with high affinity, this site is not capable of mediating transcriptional activation of transient transfection reporter constructs in mouse B lineage cells. These results therefore suggest that, in contrast to the maintenance of kappa enhancer transcription which requires all of the Eikappa sites, only the kappaB, kappaE1, and kappaE2 sites may be necessary for the recombinational activity of the enhancer. Furthermore, NF-kappaB-mediated effects on transcription and recombination appear to involve separate downstream activation pathways.