c-ErbA, but not v-ErbA, competes with a putative erythroid repressor for binding to the carbonic anhydrase II promoter.

The carbonic anhydrase II (CAII) gene is the only known gene identified as direct target for v-ErbA-mediated repression in avian erythroleukemic cells transformed by Avian Erythroblastosis Virus (AEV). This gene is transcriptionally activated by thyroid hormone (T3) in normal erythrocytic cells. In this work we have analysed the molecular basis of the transcriptional control of the CAII gene by c-ErbA and v-ErbA. We show that several domains in the promoter control hormonal regulation of transcription. One domain proximal to the TATA box mediates T3 response but contains no identified binding site for c-ErbA. An other domain termed PAL2 is approximately 600 bp upstream the transcription initiation site and contains a c-ErbA binding site. We show that when it is associated to a heterologous promoter this site mediates transcriptional repression in erythrocytic cells but not in HeLa cells. Moreover, this site binds a nuclear erythrocyte-specific factor that we called NFX, which is different from c-ErbA. heterodimers between c-ErbA and the 9-cis retinoic acid receptor (RXR) compete with NFX for binding to PAL2. In contrast, v-ErbA alone or in association with RXR is a very poor competitor and is unable to chase NFX out of the PAL2 site. We propose that NFX is a transcription repressor whose activity is inhibited by c-ErbA but not v-ErbA. This mechanism might contribute to the overall regulation of the carbonic anhydrase II promoter. These data illustrate another possible mechanism through which v-ErbA might antagonize the function of c-ErbA in controlling gene expression.