REPLICATION STRUCTURE OF THE HUMAN BETA-GLOBIN GENE DOMAIN

THE animal cell genome is organized into a series of replicons with an average size of 50-300 kilobases1; each of these units is characteriz ed by its own origin of replication which serves as the point of initi ation for DNA synthesis. In animal viruses, origin usage can be regula ted by cis-acting elements2, and in some cases, replication may be cel l-type specific3. Little is known, however, about the organization and control of endogenous tissue-specific gene replication. To understand this process, we have used a replication direction assay to examine D NA fragments covering more than 200 kilobases of the human beta-like g lobin domain, and have identified a single bidirectional origin locate d upstream of the beta-globin itself. This locus is used to initiate D NA synthesis in expressing cells, where the globin domain replicates e arly, and in non-expressing cells, which are characterized by late rep lication of the same region4,5. Deletion of this origin sequence, as o ccurs in the haemoglobin Lepore syndrome6, cancels bidirectional DNA s ynthesis at this site and leads to a striking reversal of replication direction upstream to the locus. This represents the first genetic pro of of the existence of specific, discrete origins of replication in an imal cells.