Position independence and proper developmental control of g-globin gene expression require both a 5' locus control region and a downstream sequence element

We have analyzed the expression of human-y-globin genes during development in F2 progeny of transgenic mice carrying two types of constructs. In the first type,-y.globin genes were linked individually to large (-4-kb) sequence fragments spanning locus control region (LCR) hypersensitive site 2 (HS2) or HS3. These LCR fragments contained not only the core HS elements but also extensive evolutionarily conserved flanking sequences. The second type of construct contained tandem y- and 13-globin genes linked to identical HS2 or HS3 fragments. We show that y-globin expression in transgenic mice carrying HS2-y or HS3-y constructs is highly sensitive to position effects and that such effects override the cis regulatory elements present in these constructs to produce markedly different developmental patterns of y-globin expression in lines carrying the same transgene. In contrast,-y-globin expression in both HS2-yi and HS3'yjP mice is sheltered from position effects and the developmental patterns of y-globin expression in lines carrying the same transgene are identical and display stage-specific regulation. The results suggest that cis regulatory sequences required for proper developmental control of fetal globin expression in the presence of an LCR element reside downstream from the-y genes. The,B-like globin genes in humans are organized into a locus on chromosome 11 that contains coordinately expressed em-bryonic, fetal, and adult globin genes and an upstream locus control region (LCR) (see Fig. 1) (49). Within the LCR, regulatory sequences necessary for high-level globin gene expression in cell and transgenic assays have been found to coincide with the presence of focal alterations in chromatin structure that are hypersensitive (HS) to cleavage by nuclease