We have examined whether replication of the chicken β-globin locus changes during differentiation of primary erythroid progenitors into erythrocytes. In undifferentiated progenitors, four major initiation sites and a replication fork pausing region (RFP) were observed. Forty-eight hours after induction of differentiation, the major sites were maintained, even within the activated βA-globin gene, some minor sites were enhanced, three new sites appeared and the RFP disappeared. One of the activated origins showed increased histone H3 K9K14 diacetylation, but the others did not. These results demonstrate a broadening of DNA replication origin usage during differentiation of untransformed metazoan cells and suggest that neither histone H3 diacetylation, nor other histone modifications so far reported, nor transcription, are crucial determinants of origin selection in this system.
