Cell type-specific repression of the immunoglobulin heavy chain enhancer

Abstract

The murine immunoglobulin heavy chain (IgH) enhancer was studied as a model system for enhancer function and cell type-specific gene expression. Mutational analysis revealed that the IgH enhancer is a complex transcriptional element. The enhancer is comprised of a number of sequence motifs spread over a relatively large region ($\sim$400 bp) of DNA. While all of these sequence motifs are involved in modulation of enhancer activity, none of these sites is absolutely required for enhancer function as no single mutation completely abolishes activity of the full length enhancer. Deletion analysis suggested that the cell type-specificity of the IgH enhancer is due, in part, to negative control. Deletion of a small region of the enhancer leads to enhancer activation in nonlymphoid cells. Negative regulation of cell type-specificity was confirmed in experiments characterizing the transcriptional activity of oligonucleotides containing individual IgH enhancer protein binding sites. One protein binding site, $\mu$E5, functions as a positive transcription element in lymphoid cells and a negative transcription element in nonlymphoid cells. The cell type-specificity of the IgH enhancer was negated by overexpression of the $\mu$E5-binding transcription activator, ITF-1. Presumably, ITF-1 can derepress the enhancer by out competing the repressor for binding to the $\mu$E5 site. This result suggests that the cell type-specific regulation of the IgH enhancer can be mediated through the balance of positive and negative transcription factors rather than the absolute presence or absence of these regulatory proteins. In order to further expand this model of cell type-specific gene expression a cDNA encoding a protein capable of binding specifically to the $\mu$E5 site was isolated from a nonlymphoid cDNA library