Association of autonomous replication activity with replication origins in a human chromosome

A systematic analysis of the correlation of autonomous replication activity with initiation of replication in a human chromosome was performed. The temporal order of replication of segments in a pericentric 320-kb MEN203 locus on human chromosome 10 (10q11.2) was determined by pulse-labeling of cells with 5-bromodeoxyuridine after synchronization with aphidicolin. The entire MEN203 locus replicated during the late S phase. Two distinct segments replicated earlier than the others in the locus, indicating that replication was initiated within or near these segments. Two other segments also showed an earlier response than the respective neighboring regions. These results suggest that the MEN203 locus contains two distinct replication origins and two possible origins that may be used less frequently. The results were essentially confirmed by synchronization of the cell cycle with mimosine. Analysis of autonomous replication activity of 10-kb long chromosome fragments covering the 320-kb region showed that certain fragments replicated two or three times more efficiently than others. The results are consistent with our previous observations with randomly cloned human chromosome fragments. The replication origins colocalized with fragments exhibiting relatively high autonomous replication activity. Thus, the capacity for autonomous replication of chromosome fragments might be prerequisite for the initiation of chromosomal replication.