A global profile of replicative polymerase usage

Three eukaryotic DNA polymerases are essential for genome replication. Polα-primase initiates each synthesis event and is rapidly replaced by processive DNA polymerases: Polε replicates the leading strand while Polδ performs lagging strand synthesis. However, it is not known whether this division of labour is maintained across the whole genome or how uniform it is within single replicons. Using S. pombe, we have developed a polymerase usage sequencing (Pu-seq) strategy to map polymerase usage genome–wide. Pu–seq provides direct replication origin location and efficiency data and indirect estimates of replication timing. We confirm that the division of labour is broadly maintained across an entire genome. However, our data suggest a subtle variability in the usage of the two polymerases within individual replicons. We propose this results from occasional leading strand initiation by Polδ followed by exchange for Polε.