Activation of globin genes during chicken development.

Hb first appears in the developing chicken blastoderm at 35 hr of incubation. We have isolated the presumptive Hb-forming area of the embryo at 20-23 hr and shown that most of the cells in this region are red cell precursors. The structure and activity of the globin genes in these precursors have been assayed in three ways: by runoff nuclear transcription to measure the number of initiated RNA polymerases present on the globin genes; by measuring the degree of methylation of these genes with methyl-sensitive restriction enzymes; and by monitoring the DNAase I sensitivity and the generation of DNAase I-hypersensitive sites in the globin chromatin. All these assays show the globin genes to be inactive in precursor cells. Additional experiments focusing on the mechanism of the transition from an inactive to an active chromosome structure indicate that DNA replication but not cytokinesis is intimately coupled to the transition process.