Cdc45 targeted to lacI repeats (red) leads to decondensation of the chromatin.

On page 875, Alexandrow and Hamlin report that decondensation at the replication fork appears to be triggered by phosphorylation of histone H1. The phosphorylation precedes incorporation of BrdU, and is dependent on Cdk2, perhaps explaining why Cdk2 is required for S-phase progression.To find out what happens at the fork, the team used a molecular tethering system originally designed to study higher-order chromatin remodeling and transcription (Li et al., 1998). The CHO cells used in this report contain multiple tandem copies of the lac operator stably integrated into the chromosomes. When a replication-related protein is fused to LacI protein and transfected into such cells, the replication protein is targeted to the tandem repeats because LacI binds to the lac operator.

Fusion of Cdc45, a protein associated with the replication fork itself, causes widespread decondensation of the chromatin in the system, but Cdc6, a protein required for replication initiation, does not. Moreover, Cdc45 induces phosphorylation of histone H1 by recruiting Cdk2, a protein required for entry into S phase as well as progression through it.

The group did not find evidence of acetylation or methylation changes on the core histones, which have been detected when the same experimental system was used to study transcription-induced chromatin changes. Either such changes are transient during replication—and therefore under the radar of the current experiments—or the mechanisms that underlie chromatin remodeling during replication and transcription differ.