Promoter elements of the mouse complement C4 gene critical for transcription activation and start site location.
We have explored the template and factor requirements for transcription of the gene encoding the murine complement component C4, expressed predominantly but not exclusively in liver and mononuclear phagocytes. Competition experiments in transcription assays with liver nuclear extracts show that the regions upstream of the transcription initiation site are largely dispensable for obtaining basal levels of accurately initiated transcription. Activated transcription, however, depends on three upstream regulatory factors, two of which interact with target sites seemingly related to NF-1 (region -112/-87) and USF (region -85/-64), respectively. A third upstream regulatory factor has been detected by the surprising finding that double-stranded oligomers covering sequences proximal to the cap site (position -48 to -7) stimulate transcription from the C4 promoter specifically. Results of nucleotide deletions and site-directed mutations argue that the C4 initiator, that is, the most critical element for basal and accurate transcription of the gene, overlaps the cap site and extends into the transcribed sequences (-1 to +12). Immediately downstream of this region lies a last regulatory element (within the +5 to +43 boundaries) indispensable for high levels of transcription. These data assume wider interest because the C4 promoter does not contain TATA or CAAT boxes and does not feature any of the elements characteristic of the TATA-less genes so far reported.