RECOGNITION OF THE E-C4 ELEMENT FROM THE C4 COMPLEMENT GENE PROMOTER BY THE UPSTREAM STIMULATORY FACTOR-I TRANSCRIPTION FACTOR

Activation of complement gene expression plays a major role in the res ponse to antigenic challenge. The induction of complement synthesis oc curs primarily in liver and in macrophages and is mediated, at least i n part, by increased transcription of the complement genes. For exampl e, transcription of the C4 complement gene, which plays a crucial role in the complement pathway, is induced in response to acute inflammati on or tissue injury. Previous work has defined the elements present in the C4 complement gene promoter that are required for its expression. Particularly important is an E-box motif, E-C4, that is conserved bet ween the mouse, human, and rat promoters and that directed up to 90% o f transcription from the mouse C4 promoter. Here we have purified the E-C4-binding factor to homogeneity using a novel and rapid affinity pu rification procedure. Following N-terminal microsequencing and subsequ ent isolation of the corresponding cDNA, the factor binding the E-C4 e lement was identified as upstream stimulatory factor-1 (USF-1), a basi c helix-loop-helix-leucine zipper transcription factor. We also show f or the first time that in vivo USF-I is a phosphoprotein, but that pho sphorylation of USF-1 is severely reduced in cells in culture. Moreove r, the phosphorylated form of USF-1 binds DNA preferentially, indicati ng that phosphorylation may enhance the ability of USF-1 to bind DNA. The implications of USF-1 phosphorylation for C4 complement gene expre ssion and transcription regulation are discussed.