An enhancer/locus control region is not sufficient to open chromatin.

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Abstract
To study the way in which an enhancer/locus control region (LCR) activates chromatin, we examined transgenic mice carrying various combinations of the chicken beta A-globin gene coding region, promoter, and 3' enhancer/LCR. We compared lines carrying only the coding region and enhancer R (E) and only the coding region and promoter (P) with those containing all three elements (PE). We have shown previously that all PE mice transcribe the transgene in a copy number-dependent manner while the P mice do not express their transgene. In the current study, we examined chromatin activation by monitoring formation of erythroid-specific hypersensitive sites at the promoter and enhancer. We found that all of the PE lines but none of the P lines show hypersensitivity. In contrast, only three of six E lines are hypersensitive (two strongly and one weakly), demonstrating position dependence of this transgene. The two E lines with strong hypersensitive sites were found also to have RNA complementary to the transgene, presumably starting from an adjacent adventitious mouse promoter. In all of these lines, we found a correlation between erythroid-specific hypersensitivity and erythroid-specific general DNase I sensitivity, an indicator of regional chromatin activation. The results support a mutual interaction model for the mechanism of chromatin opening by LCRs in which the enhancer/LCR and promoter must cooperate in order to generate open chromatin. The data are not consistent with a dominant enhancer model in which the enhancer/LCR can open chromatin autonomously.