The PU.1 and NF-EM5 binding motifs in the Ig kappa 3 ' enhancer are responsible for directing somatic hypermutations to the intrinsic hotspots in thetransgenic V-kappa gene

Somatic hypermutation is a key mechanism in generating Ig with higher affin ities to antigen, a process known as affinity maturation. Using Ig kappa tr ansgenes, the 3' enhancer (kappa E3') has been shown to play an important r ole in introducing hypermutations. In order to identify the cis-acting elem ents that regulate hypermutagenesis, we have generated transgenic substrate s containing mutations/deletions in the kappa E3' region. Here, we report t hat base substitutions in the kappa E3', either in the PU.1 or in the NF-EM 5 binding motif, not only reduce the mutation rate but also disrupt the dir ected mutagenesis in the intrinsic hotspots of the Ig kappa transgene.