Upstream stimulatory factor regulates major histocompatibility complex class I gene expression: the U2 Delta E4 splice variant abrogates E-box activity

The tissue-specific expression of major histocompatibility complex class I genes is determined by a series of upstream regulatory elements, many of wh ich 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 ce ll type-specific enhancer of class I promoter activity. The class I E box i s very active in a neuroblastoma cell line, CHP-126, but is relatively inac tive in the HeLa epithelial cell line. The basic helix-loop-helix leucine z ipper proteins upstream stimulatory factor 1 (USF1) and USF2 were shown to specifically recognize the class I E box, resulting in the activation of th e downstream promoter. Fine mapping of USF1 and USF2 amino-terminal functio nal domains revealed differences in their abilities to activate the class I E box. Whereas USF1 contained only an extended activation domain, USF2 con tained both an activation domain and a negative regulatory region. Surprisi ngly, the naturally occurring splice variant of USF2 lacking the exon 4 dom ain, U2 Delta E4, acted as a dominant-negative regulator of USF-mediated ac tivation of the class I promoter. This latter activity is in sharp contrast to the known ability of U2 Delta E4 to activate the adenovirus major late promoter. Class I E-box function is correlated with the relative amount of U2 Delta E4 in a cell, leading to the proposal that U2 Delta E4 modulates c lass I E-box activity and may represent one mechanism to fine-tune class I expression in various tissues.