The -globin locus control region (LCR) is necessary for high-level -globin gene transcription and differentiation-dependent relocation of the -globin locus from the nuclear periphery to the central nucleoplasm and to foci of hyperphosphorylated Pol II "transcription factories" (TFys). To determine the contribution of individual LCR DNaseI hypersensitive sites (HSs) to transcription and nuclear location, in the present study, we compared -globin gene activity and location in erythroid cells derived from mice with deletions of individual HSs, deletions of 2 HSs, and deletion of the whole LCR and found all of the HSs had a similar spectrum of activities, albeit to different degrees. Each HS acts as an independent module to activate expression in an additive manner, and this is correlated with relocation away from the nuclear periphery. In contrast, HSs have redundant activities with respect to association with TFys and the probability that an allele is actively transcribed, as measured by primary RNA transcript FISH. The limiting effect on RNA levels occurs after -globin genes associate with TFys, at which time HSs contribute to the amount of RNA arising from each burst of transcription by stimulating transcriptional elongation.
IntroductionAnalysis of naturally occurring and targeted deletions of the endogenous -globin locus control region (LCR) and of transgenic mice has revealed that the LCR is required for high-level -globin transcription in erythroid cells. [1][2][3] The LCR DNase1 hypersensitive sites (HSs) contain similar, albeit distinct sequence motifs, and vary in their activities in vitro and in transgenic mouse assays. [2][3][4][5][6][7][8][9][10] Therefore, each HS may play a unique role in LCR function or may contribute a similar spectrum of activities. Transgenic mice bearing individual LCR HSs, combinations of HSs, or an intact LCR linked to the human -globin locus have led to models in which the LCR HSs interact to form a holocomplex in which the individual HSs 1-4 act synergistically to ensure a permissive chromatin environment and activate expression, whereas 5ЈHSs 5 and 6 act as a chromatin barrier. 5,[11][12][13][14][15][16][17][18][19] In addition, it has been suggested that 5ЈHS2 has a dominant or unique activity compared with other LCR HSs. In contrast to these transgenic studies, our analysis of the transcriptional phenotypes of mice in which each LCR HS was deleted individually from the endogenous locus suggested that each site might contribute additively to LCR-mediated gene activation. [20][21][22] The LCR has also been implicated in the localization of the -globin locus during erythroid differentiation. During differentiation, the -globin locus relocates away from the nuclear periphery and increasingly associates with foci of hyperphosphorylated Pol II "transcription factories" (TFys), where high-level transcription is activated. 23 Deletion of the HSs that comprise the endogenous mouse -globin LCR results in both decreased relocation of the locus to the nucleoplasm and asso...