Molecular cloning and structural analysis of human sterol C5 desaturase.

Sterol C5 desaturase (SC5D) converts lathosterol to 7-dehydrocholesterol in cholesterol biosynthesis. In this study, we investigated the genome structure of SC5D and transcription of the human SC5D gene to try to elucidate the mechanism by which cholesterol synthesis is regulated. The SC5D gene had a structural gene from a single copy from genome DNA that contained five exons and four introns. The human SC5D was found to be located in chromosome 11q24.2-24.3 by fluorescence in situ hybridization mapping. Human SC5D mRNA transcripts, of which the major transcript was a 2 kb and the minor transcripts were 8 kb and 1.4 kb mRNA transcripts, were detected in almost all of the tissues examined. The human SC5D gene contained a GC box instead of a TATA box upstream of the transcript start sites. Human SC5D transcription started from several transcription start sites, and the first start site was located 31 bp upstream of the translation start site (ATG). The expression level of SC5D mRNA extracted from human liver carcinoma cells decreased as the amount of cholesterol added to the culture medium was increased. Inhibition of SC5D transcription was ascribed to the suppression of promoter activity of SC5D.