REVERSAL OF TERMINAL DIFFERENTIATION AND CONTROL OF DNA-REPLICATION -CYCLIN-A AND CDK2 SPECIFICALLY LOCALIZE AT SUBNUCLEAR SITES OF DNA-REPLICATION

DNA replication in mammalian cells occurs in discrete nuclear foci. He re we show that terminally differentiated myotubes can be induced to r eenter S phase and show the same pattern of replication foci as cyclin g cells. We used this cellular system to analyze the interaction of ce ll cycle proteins with these foci in vivo. Cyclin A and cdk2, but not cyclin B1 and cdc2, were specifically localized at nuclear replication foci, just like the replication protein proliferating cell nuclear an tigen. A potential target of cyclin A and cdk2 is the 34 kd subunit of replication protein A (RPA34). In contrast with the 70 kd subunit, wh ich localizes to the foci, RPA34 was not detected at these replication sites, which may reflect a transient interaction. The specific locali zation of cyclin A and cdk2 at nuclear replication foci provides a dir ect link between cell cycle regulation and DNA replication.