Expression of the dibasic proprotein processing enzyme furin is directed by multiple promoters

The prototype mammalian proprotein processing enzyme furin is shown to be encoded by three distinct FUR mRNA isoforms which differ only in their 5'-untranslated regions. By primer extension analysis, the transcription start sites of the three mRNA isoforms were defined. The genomic regions located immediately upstream of the three alternative transcriptional start sites were shown to possess promoter activity in transfection experiments using the luciferase encoding gene as reporter. In a liver cell line, the P1 promoter appeared to be the strongest; in a lung cell line, the P1A promoter. Human FUR promoter P1 but not P1A or P1B was transactivated by transcription factor C/EBP beta. Other members of this family of bZIP transcription factors, C/EBP alpha and C/EBP delta, were not able to transactivate the P1 promoter. Promoter P1A and P1B have characteristics of promoters of housekeeping genes. They lack TATA or CAAT boxes upstream of the transcription start site but are very GC-rich and contain several SP1 sites. Promoter P1, on the other hand, has a TATA box in the proximal promoter region. In electromobility shift assays and DNase I footprinting analysis, transcription factor SP1 was found to bind to the proximal region of the P1 promoter. Altogether, our results indicate that expression of the human FUR gene is directed by alternative promoters, housekeeping (GC-rich) as well as regulated (TATA-containing) promoters, suggesting that their differential use may be a mechanism to modulate levels of the furin enzyme.