The C/EBP family of transcriptional activators is functionally important for Ig VH promoter activity in vivo and in vitro.

We have examined the functional importance of binding sites for C/EBP family members (E sites), in two Ig VH promoters: VH1, a member of the S107 family, and BCL1, a member of the J558 family. Mutation of the E site in the VH1 promoter diminishes transcription in vivo to 59% of wild-type and transcription from the BCL1 promoter in vitro is inhibited to an average of 39% of wild-type by competition with E site oligonucleotides. Purified E site binding proteins from plasmacytoma cells stimulated BCL1 transcription in vitro 2.3-fold. Although five C/EBP family proteins are known which bind the E site, antibody ablation of DNA:protein complexes resolved by electrophoretic mobility shift assays showed that Ig/EBP-1 is the only E site binding protein detectable in early B cell lines; more mature B cells contain Ig/EBP-1 and NF-IL6. We also show by antibody-depletion that Ig/EBP-1 activates the BCL1 promoter in vitro.