THE NONHISTONE CHROMOSOMAL PROTEIN HMG-I(Y) CONTRIBUTES TO REPRESSIONOF THE IMMUNOGLOBULIN HEAVY-CHAIN GERM-LINE EPSILON-RNA PROMOTER

The rate of germ-line RNA transcription correlates with the rate of im munoglobulin heavy chain isotype switching. A promoter element for the transcription of RNA from the germ-line mouse immunoglobulin epsilon heavy chain constant region gene is induced by interleukin(IL)-4 and l ipopolysaccharide, and is bound at its transcription initiation sites by an IL-4-inducible nuclear protein, NF-BRE. To examine the function of the binding site for this IL-4-inducible complex, substitution muta tions were introduced in the promoter. These binding site mutations in creased promoter activity and decreased binding of NF-BRE. To investig ate the paradox of an IL-4-inducible protein binding to a repressor si te in an IL-4-inducible promoter, we determined that the non-histone c hromosomal protein HMG-I(Y) binds at the transcription initiation site s of the germ-line epsilon promoter. Assays with antisera against HMG- I(Y) revealed monomeric HMG-I(Y) in nuclear extracts. Cotransfection o f an expression construct directing the synthesis of anti-sense HMG-I( Y) RNA also increased promoter activity, consistent with a repressor f unction of HMG-I(Y). Thus, the data are most consistent with a model i n which HMG-I(Y) participates in repression of promoter activity. The effects of IL-4 may include derepression at this site.