AN ENHANCER LOCUS-CONTROL REGION IS NOT SUFFICIENT TO OPEN CHROMATIN

To study the way in which an enhancer/locus control region (LCR) activ ates chromatin, we examined transgenic mice carrying various combinati ons of the chicken beta(A)-globin gene coding region, promoter, and 3' enhancer/LCR. We compared lines carrying only the coding region and e nhancer/LCR (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 wh ile the P mice do not express their transgene. In the current study, w e examined chromatin activation by monitoring formation of erythroid-s pecific hypersensitive sites at the promoter and enhancer. We found th at all of the PE lines but none of the P lines show hypersensitivity. In contrast, only three of six E lines are hypersensitive (two strongl y 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 a djacent 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 chroma tin 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 enhance r/LCR can open chromatin autonomously.