Polarity of DNA replication through the avian alpha-globin locus

Toward understanding the controls affecting eucaryotic chromosome replication, we used a runoff replication assay to investigate whether the activity of a gene is related to its use of an upstream or downstream replication origin. When in vivo-initiated DNA polymerases are allowed to complete replication in vitro in the presence of bromodeoxyuridine triphosphate the density label is preferentially incorporated into origin-distal regions of DNA. Isopycnic centrifugation and blot hybridization analysis of the relative bromodeoxyuridine triphosphate incorporation into fragments spanning the chicken alpha-globin locus indicate that this region is replicated from an upstream origin both in chicken lymphocytes and in erythrocytes. Thus the replication polarity of these genes does not change as a function of transcriptional activity, consistent with earlier suggestions that DNA replication in the transcriptional direction may be a necessary but not sufficient condition for gene expression.