CHARACTERIZATION OF THE MOUSE FTZ-F1 GENE, WHICH ENCODES A KEY REGULATOR OF STEROID HYDROXYLASE GENE-EXPRESSION

The cytochrome P450 steroid hydroxylases are coordinately regulated by steroidogenic factor 1 (SF-1), a protein expressed selectively in steroidogenic cells. Based on its expression in steroidogenic tissues and DNA-binding specificity, we isolated a putative SF-1 cDNA from an adrenocortical cDNA library. As evidence that this cDNA encodes SF-1, we now show that it is selectively expressed in steroidogenic cells, that an antiserum against its protein product specifically abolishes the SF-1-related gel-shift complex, and that its coexpression increases promoter activity of the 21-hydroxylase 5'-flanking region in transfection experiments. Sequence analyses of the SF-1 cDNA revealed that it is the mouse homolog of fushi tarazu factor I (FTZ-F1), a nuclear receptor that regulates the fushi tarazu homeobox gene in Drosophila. A second FTZ-F1 homolog, embryonal long terminal repeat-binding protein (ELP), was recently isolated from embryonal carcinoma cells. SF-1 and ELP cDNAs are virtually identical for 1017 base pairs, including putative DNA-binding domains, but diverge at their 5'- and 3'-ends. One genomic clone contained both SF-1- and ELP-specific sequences, confirming their origin from a single gene. Characterization of this gene defined shared exons encoding common regions and alternative promoters and 3'-exons leading to differences between the two FTZ-F1 transcripts. We used in situ hybridization with transcript-specific probes to study the ontogeny of SF-1 and ELP expression. ELP transcripts were not detected from embryonic day 8 to adult, consistent with its previous isolation from embryonal carcinoma cells and its postulated role in early embryonic development. In contrast, SF-1 transcripts were first detected at embryonic day 12, when steroidogenic organs begin to develop and steroidogenic enzymes are first expressed, and persisted thereafter at high levels in adrenal glands and testis. Collectively, these studies establish that the mouse homolog of the FTZ-F1 gene generates developmentally regulated transcripts, one of which encodes an essential regulator of steroid hydroxylase gene expression.