Upstream Stimulatory Factor Regulates Major Histocompatibility Complex Class I Gene Expression: the U2ΔE4 Splice Variant Abrogates E-Box Activity

Abstract

The tissue-specific expression of major histocompatibility complex class I genes is determined by a series of upstream regulatory elements, many of which remain ill defined. We now report that a distal E-box element, located between bp −309 and −314 upstream of transcription initiation, acts as a cell type-specific enhancer of class I promoter activity. The class I E box is very active in a neuroblastoma cell line, CHP-126, but is relatively inactive in the HeLa epithelial cell line. The basic helix-loop-helix leucine zipper proteins upstream stimulatory factor 1 (USF1) and USF2 were shown to specifically recognize the class I E box, resulting in the activation of the downstream promoter. Fine mapping of USF1 and USF2 amino-terminal functional domains revealed differences in their abilities to activate the class I E box. Whereas USF1 contained only an extended activation domain, USF2 contained both an activation domain and a negative regulatory region. Surprisingly, the naturally occurring splice variant of USF2 lacking the exon 4 domain, U2ΔE4, acted as a dominant-negative regulator of USF-mediated activation of the class I promoter. This latter activity is in sharp contrast to the known ability of U2ΔE4 to activate the adenovirus major late promoter. Class I E-box function is correlated with the relative amount of U2ΔE4 in a cell, leading to the proposal that U2ΔE4 modulates class I E-box activity and may represent one mechanism to fine-tune class I expression in various tissues.

ACKNOWLEDGMENTS

We gratefully acknowledge Carol Thiele, Stephen Straus, Barbara L. Rellahan, Shelby Berger, and Julie Brown for valuable suggestions and discussions. We thank Robert Roeder for providing the U2E1bCAT reporter construct.

S.J.H. was supported by the HHMI Summer Research Program.