A conserved sequence element situated between the decamer and TATA box in V kappa II and V kappa V promoters has a high homology to the binding site for early B cell factor (EBF). The kappa promoter element was shown to bind EBF specifically using both in vitro-translated protein and nuclear extract. Concomitant binding of EBF and Oct proteins to a wild-type kappa promoter template was observed at low efficiency, and such dual occupancy was dependent on an intact amino terminus of the Oct protein. When the two binding sites were separated by a 10-bp spacer, this dependency disappeared. A single kappa promoter EBF site together with a TATA box and an Ig heavy chain enhancer showed marginal transcriptional stimulatory activity. In contrast, the EBF site acted synergistically with a decamer element in EBF-negative plasmacytoma cells, but not in B cells of an earlier differentiation stage. In these cells, a distinct protein was observed that interacted with the EBF binding motif, while overexpression of EBF down-regulated the expression of a reporter construct containing Ig control elements.

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