EXPRESSION OF THE HUMAN CYTOCHROME C1 GENE IS CONTROLLED THROUGH MULTIPLE SP1-BINDING SITES AND AN INITIATOR REGION

It is widely accepted that nuclear genes that encode proteins of the o xidative-phosphorylation system are regulated by nuclear factors belie ved to be specific for such genes. In the present study we show that t he promoter for the human cytochrome c1 gene is an exception, in that it involves only conserved Sp1 core elements and an initiator region. Maximal promoter activity within a 1.4-kb 5' flanking region of the cy tochrome cl gene is contained in a fragment (-72 to +18) that lacks TA TA and CCAAT elements. The transcriptional start site was mapped to an initiator region by RNase protection of mRNA from human HepG2 cells, and by primer extension of in vitro-generated transcripts, to a sequen ce that is highly similar to the dihydrofolate reductase family of ini tiators. Deletion of this region (+1 to +18) severely impairs transcri ption initiation. Sp1 core elements centered at nucleotides -21 and -3 9 define the activation domain of the proximal promoter. Only the -39 element is protected from DNase I in the presence of crude nuclear ext racts. However, transfection, gel-mobility-shift, supershift and in vi tro-transcription experiments show that the -21 element binds Spl prot ein and contributes to transcription activation. No other function al oxidative-phosphorylation-specific response elements have been identif ied. These data implicate Sp1 as a single activating factor for an oxi dative-phosphorylation gene.