Dual promoter activation by the human beta-globinlocus control region.

The human beta-globin locus control region (LCR)is necessary for high-level and position-independent expression of globin genesin erythroid cells. A variety of mechanisms have been proposed for thecis-activation of individual members of the beta-globin gene family by the LCRlocated 10-50 kilobases upstream. It is not known, however, whether a given LCRcan activate all developmentally appropriate globin family members on itschromosome or whether, within a given chromosome, the LCR must be committed toactivating only a single gene. We have devised an experiment to distinguishbetween these possibilities. This experiment takes advantage of the fact that iftwo genes in a cluster are transcriptionally active and their promoters,therefore, are in a conformation hypersensitive to nucleases, restrictionenzymes that cleave the promoters will excise the intervening chromatinfragment. The Apa I sites on human fetal G gamma- and A gamma-globin genepromoters are accessible to cleavage in nuclei from the human erythroleukemiacell line K562, which expresses these genes, but not in HeLa cells. We find thatApa I digestion leads to excision in high yield of the fragment spanning thesepromoters, showing that a LCR element is capable of sharing its activatingfunction among members of a gene cluster on a single chromosome.