Regulation of Transcription of the TATA-less Human Complement Component C4 Gene

Abstract

AbstractThe 5′-sequences flanking the human complement component C4 genes (C4A and C4B) have been analyzed for their ability to direct expression of a reporter gene in cell lines that constitutively express or do not express C4. No difference in the level of reporter gene expression was detected in cells transfected with C4A- or C4B-specific constructs. A series of reporter constructs containing progressively truncated C4 promoter fragments transfected into the hepatocyte Hep G2 cell line, identified the sequence contained within the region −178 to −39 as that associated with maximal reporter gene expression. This region contains consensus binding motifs for nuclear factor 1 (−110 to −97), Sp1 (−57 to −49), and three basic helix-loop-helix (−137 to −132, −98 to −93, and −78 to −73)-like transcription factors. Electromobility shift assays and DNase I footprinting analysis showed specific DNA-protein interactions of the C4 promoter at the nuclear factor 1, two E box (−98 to −93 and −78 to −73), and Sp1 binding domains. Site-directed mutagenesis of the Sp1 binding site resulted in total abrogation of reporter gene expression and mutation of the E box (−78 to −73) resulted in a 8-fold reduction in expression. We conclude that the Sp1 binding site at position −57 to −49 is critical for accurately initiated, basal transcription of C4.