INDEPENDENT PATHWAYS FOR DE-REPRESSION OF THE MOUSE IG HEAVY-CHAIN GERM-LINE-EPSILON PROMOTER - AN IL-4 NAF NF-IL-4 SITE AS A CONTEXT-DEPENDENT NEGATIVE ELEMENT/

The activation of germ-line promoters in the Ig heavy chain loci is re gulated by cytokines as part of the regulation of B cell commitment to production of new antibody isotypes. Activation of the germ-line prom oter of the epsilon heavy chain locus (G epsilon) and production of Ig E are induced by IL-4 and each is virtually undetectable in the absenc e of IL-4 or the homologous cytokine IL-13. Basal expression of the G epsilon promoter is repressed by the non-histone chromosomal protein H MG-I(Y), which also contributes to promoter inducibility, and IL-4 sti mulates phosphorylation of the C-terminus of HMG-I(Y) through a rapamy cin-sensitive pathway. IL-4 treatment of mouse B cells also induces a G epsilon DNA binding activity with the properties of IL-4 NAF, which is rapidly induced and requires phosphotyrosine for DNA binding activi ty. This protein binds to a different site from HMG-I(Y), but the IL-4 NAF/NF-IL-4 binding site also is a negative element more active in re pression of basal transcription of the GE promoter. This site acts as a negative element when transferred to the thymidine kinase promoter, but does not confer inducibility. In contrast to HMG-I(Y), IL-4 NAF/NF -IL-4 activation is refractory to rapamycin but sensitive to genistein . These findings indicate that two independent signal transduction pat hways diverge from the IL-4 receptor and suggest that normal expressio n of G epsilon RNA or IgE is low in part because the germ-line promote r is kept in a state of repression which requires de-repression throug h several cooperative pathways. These pathways target conserved nucleo tide sequence motifs whose precise function depends on the promoter co ntext in which they are situated.