The chicken beta/epsilon-globin enhancer directs autonomously regulated, high-level expression of the chicken...

In transiently transfected chicken erythroid cells, beta-like globin gene switching is mediated through differential activation of the cis-linked embryonic epsilon- and adult beta-globin genes by a shared enhancer. Two underlying mechanisms have been proposed: (i) tissue- and stage-specific factors activate the beta-globin promoter in adult erythroid cells (autonomous regulation); and (ii) the epsilon-globin promoter, although transcriptionally competent in both embryonic and adult cells, is suppressed at the adult stage through competition with the beta-globin promoter for interaction with the enhancer (competitive regulation). Analyses of transgenic mice carrying the chicken beta/epsilon-globin locus demonstrated that both genes depended on the enhancer for erythroid expression, but only the epsilon-globin gene exhibited developmentally appropriate transcription at levels comparable to the endogenous mouse globin genes. Surprisingly, the chicken epsilon-globin gene also appeared to be autonomously regulated, as has been observed for human embryonic and fetal beta-like globin genes in transgenic mice. These results suggest that the chicken beta/epsilon-globin enhancer possesses either embryonic stage or epsilon-globin gene specificity when incorporated into the murine germ line.