Hemoglobin switching is an example of tissue and temporal regulation of gene expression. Embryonic globin chains (zeta and epsilon) are expressed during the first trimester of gestation. Fetal globin genes (alpha and gamma) are the predominant globin chains expressed during the second and third trimester of birth. After birth alpha and beta globin are the predominant globin chains expressed. There is evidence that competition for trans-acting factors is important for appropriate developmental regulation of gamma and beta globin gene expression; however, epsilon globin gene expression exhibits appropriate developmental regulation even in the absence of other beta-like globin genes (gamma and beta) indicating the competition for factors with the beta and gamma gene is not required for appropriate developmental regulation of epsilon globin. The epsilon globin gene and flanking DNA and trans-acting factors are important for autonomous regulation of epsilon gene expression. The epsilon globin silencer has been shown to be important for autonomous regulation of epsilon. Previously two factors have been shown to bind to the silencer, an erythroid specific and ubiquitous factor. We have begun characterizing the activities of these factors. We have evidence that one factor is a transcription activator and that the other is a transcription inhibitor. Using mobility shift assays we have also shown that these factors bind to overlapping regions and likely compete for binding to the silencer. We are in the process of purifying these factors using protein chemistry and molecular genetic approaches.
