INTERACTION OF SEVERAL RELATED GC-BOX-BINDING AND GT-BOX-BINDING PROTEINS WITH THE INTRONIC ENHANCER IS REQUIRED FOR DIFFERENTIAL EXPRESSION OF THE GB110-GENE IN EMBRYONAL CARCINOMA-CELLS

The molecular mechanisms by which expression of a gene is down-regulat ed after differentiation of F9 embryonal carcinoma cells into parietal endoderm-like cells was studied by characterizing the cis- and trans- regulatory elements of the gb110 gene. This gene encodes a putative RN A helicase, and its expression is down-regulated when F9 cells are dif ferentiated with retinoic acid and cyclic AMP. The 5'-flanking region of the gene has all of the features of a GC-rich island promoter and s eems to play only a minor role, if any, in the regulated expression. A 133-bp enhancer in the first intron was identified by transient chlor amphenicol acetyltransferase assays that activated expression in undif ferentiated F9 cells about 50- to 100-fold. As this enhancer was not a ctive in differentiated F9 cells, it seems to be the prime mediator of the differentiation-specific down-regulation of the gb110 gene. Four different protein-binding sites, three of which contain GC- and GT-box motifs, were identified in the enhancer element. The fourth site, int eracting with previously described transcription factor FTZ-F1/ELP, se ems to be of minor importance for the activity of the enhancer. Mutati onal analysis showed that the cooperative interaction of several most likely related proteins,vith the three GC- and GT-bus motifs was requi red for full enhancer activity. On the basis of their binding properti es, at least two of these proteins seem to be identical or closely rel ated to ubiquitous transcription factor Spl. One of the GT-box-binding proteins was present in undifferentiated F9 cells hut not, however, i n its differentiated derivatives. The cell specificity of this transcr iption factor explains why the gb110 gene is not expressed or expresse d only at loa levels in parietal endoderm-like cells.