Distinct negative regulatory mechanisms involved in the repression of human embryonic epsilon- and fetal G gamma-globin genes in transgenic mice

Molecular aspects of embryonic hemoglobin function

In order to provide the appropriate level of oxygen transport to respiring tissues, we need to produce a molecular oxygen transporting system to supplement oxygen diffusion and solubility. This supplementation is provided by hemoglobin. The role of hemoglobin in providing oxygen transport from lung to tissues in the adult is well-documented and functional characteristics of the fetal hemoglobin, which provide placental oxygen exchange, are also well understood. However the characteristics of the three embryonic hemoglobins, which provide oxygen transport during the first three months of gestation, are not well recognized. This review seeks to describe the state of our understanding of the temporal control of the expression of these proteins and the oxygen binding characteristics of the individual protein molecules. The modulation of the oxygen binding properties of these proteins, by the various allosteric effectors, is described and the structural origins of these characteristics are probed.

Mutation of gene-proximal regulatory elements disrupts human epsilon-, gamma-, and beta-globin expression in yeast artificial chromosome transgenic mice

Previous studies have defined transcriptional control elements, in addition to the promoters, that both lie near individual human beta-globin locus genes and have been implicated in their differential stage-specific regulation during development (i.e., are believed to directly participate in hemoglobin switching). We have reinvestigated the activities during erythropoiesis that might be conferred by two of the more intensively analyzed of these elements, the epsilon-globin gene 5' silencer and the beta-globin gene 3' enhancer, by deleting them from a yeast artificial chromosome that spans the human beta-globin locus, and then analyzing transgenic mice for expression of all of the human genes. These studies show that sequences within the epsilon-globin "silencer" are not only required for silencing but are also required for activation of epsilon-globin transcription; furthermore, deletion of the silencer simultaneously reduced gamma-globin transcription during the yolk sac stage of erythroid development. Analysis of the adult beta-globin gene 3' enhancer deletion showed that its deletion affects only that gene.