Developmental aspects of chromatin structure and gene expression.

We have attempted to investigate the conformational basis and developmental relevance of globin-associated hypersensitive sites in avian and human cells. Our results indicate that once formed, DNaseI-hypersensitive sites have the capacity to propagate their own structure to daughter cells in the absence of the original events that have led to their formation. We postulate that the single-stranded character (as revealed by S1 nuclease) of these DNaseI-hypersensitivity sites could explain these results. In addition, we show that the locations of hypersensitive sites in an active avian endogenous retrovirus remain unmethylated in mature sperm and suggest that this phenomenon could lead to the propagation of structural information from germ line to fertilized egg. We have also investigated the chromatin structure of the chromosomal DNA regions containing the human G gamma-, A gamma-, delta-, and beta-globin structural genes in both fetal and adult erythropoietic tissues. Our results indicate that DNaseI introduces specific cuts into the beta-globin gene cluster in erythroid cells, but not in white blood cells. The predominant sites are located at the 5' sides of the G gamma-, A gamma-, delta-, and beta-globin genes, within 200 bp of the respective CAP sites. Examination of fetal liver cells has revealed the presence of hypersensitive sites at the 5' side of all four genes, whereas analysis of adult bone marrow has revealed the characteristic sites near the delta- and beta-globin genes but no hypersensitive sites at the 5' termini of the G gamma- or A gamma-globin genes. The presence of delta- and beta-hypersensitive sites in fetal cells suggests that the increment in expression of the delta and beta genes during development most likely involves the modulation of another pathway to gene expression.