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Differential Regulation of Mouse Germline Ig γ1 and ε Promoters by IL-4 and CD401

Before Ig class switching, RNA transcription through the specific S regions undergoing recombination is induced by cytokines and other activators that induce and direct switching. The resulting germline (GL) transcripts are essential for switch recombination. To understand the differential regulation of mouse IgG1 and IgE, we compared the promoters for GL γ1 and ε transcripts. We addressed the question of why the promoter that regulates GL ε transcription is more responsive to IL-4 than the γ1 promoter and also why GL ε transcription is more dependent on IL-4 than is γ1 transcription. We found that the IL-4-responsive region of the GL ε promoter is more inducible than that of the γ1 promoter, although each promoter contains a binding site for the IL-4-inducible transcription factor Stat6, located immediately adjacent to a binding site for a basic region leucine zipper (bZip) family protein. However, the arrangement and sequences of the sites differ between the ε and γ1 promoters. The GL ε promoter binds Stat6 with a 10-fold higher affinity than does the γ1 promoter. Furthermore, the bZip elements of the two promoters bind different transcription factors, as the GL ε promoter binds and is activated by AP-1, whereas the γ1 promoter binds and is activated by activating transcription factor 2. C/EBPβ and C/EBPγ also bind the γ1 bZip element, although they inhibit rather than activate transcription. However, inhibition of promoter activity by C/EBPβ does not require the bZip element and may instead occur via inhibiting the activity of NF-κB.