A gas pedal for replication?

PCNA is monoubiquitylated during replication of undamaged DNA in frog eggs, report Leach and Michael on page 947, and preventing the modification slows fork progression. Thus, cells may have a mechanism for modulating the speed of replication, an idea that runs counter to long-standing dogma.

Homotrimers of unmodified PCNA clamp onto DNA during replication and increase the processivity of polymerase δ, which is the workhorse of normal replication. In response to DNA damage, PCNA is monoubiquitylated in mammalian cells and mono- and polyubiquitylated in yeast. The modified protein assists in recruitment of polymerase η to sites of damage.

Leach and Michael found that Xenopus PCNA was monoubiquitylated or sumoylated during replication of undamaged DNA in egg extracts. The protein was polyubiquitylated in the presence of damaged DNA. Sumoylation was not required for replication, though...