Erythroid-specific activation and derepression of the chick beta-globin promoter in vitro.

Transcriptionally active extracts were prepared from chick red cells isolated at different stages of development. The template activity of cloned beta-globin genes is highest in extracts from definitive red cells, where the endogenous gene is normally expressed, and lowest in extracts from primitive red cells or nonerythroid tissues. This system has been used to identify regulatory elements and to assign functions to the proteins that bind within the beta-globin promoter. Regulation of expression is achieved, in part, by factors whose composition changes during red cell development. Two proteins, PAL and CON, bind at adjacent sites but have opposite effects on transcription in vitro. Levels of PAL, a potent repressor, are highest in mature red cells while those of CON, an activator, are highest in actively transcribing red cells. The effect of PAL can be overcome by blocking its binding site with a protein having a similar recognition sequence but a dissimilar function.