DUAL PROMOTER ACTIVATION BY THE HUMAN BETA-GLOBIN LOCUS-CONTROL REGION

The human beta-globin locus control region (LCR) is necessary for high -level and position-independent expression of globin genes in erythroi d cells. A variety of mechanisms have been Proposed for the cis-activa tion of individual members of the beta-globin gene family by the LCR l ocated 10-50 kilobases upstream. It is not known, however, whether a g iven LCR can activate all developmentally appropriate globin family me mbers on its chromosome or whether, within a given chromosome, the LCR must be committed to activating only a single gene. We have devised a n experiment to distinguish between these possibilities. This experime nt takes advantage of the fact that if two genes in a cluster are tran scriptionally active and their promoters, therefore, are in a conforma tion hypersensitive to nucleases, restriction enzymes that cleave the promoters will excise the intervening chromatin fragment. The Apa I si tes on human fetal (G) gamma- and (A) gamma-globin gene promoters are accessible to cleavage in nuclei from the human erythroleukemia cell l ine K562, which expresses these genes, but not in HeLa cells. We find that Apa I digestion leads to excision in high yield of the fragment s panning these promoters, showing that a LCR element is capable of shar ing its activating function among members of a gene cluster on a singl e chromosome.