The matrix attachment region in the Chinese hamster dihydrofolate reductase origin of replication may be required for local chromatid separation.

Centered in the Chinese hamster dihydrofolate reductase origin of replication is a prominent nuclear matrix attachment region (MAR). Indirect lines of evidence suggested that this MAR might be required for origin activation in early S phase. To test this possibility, we have deleted the MAR from a Chinese hamster ovary variant harboring a single copy of the dihydrofolate reductase locus. However, 2D gel replicon mapping shows that removal of the MAR has no significant effect either on the frequency or timing of initiation in this locus. Rather, fluorescence in situ hybridization studies on cells swollen under either neutral or alkaline conditions show that deletion of the MAR interferes with local separation of daughter chromatids. This surprising result provides direct genetic evidence that at least a subset of MARs performs an important biological function, possibly related to chromatid cohesion and separation.