oriX: a new replication origin in E. coli.

Replication of the chromosome of E. coli at 42 degrees C in an integratively suppressed dnaA mutant (dnaA46 Sin Hfr) occurs predominantly from the origin of replication of the integrated plasmid (oriV). We have carried out a detailed marker frequency analysis on such Hfrs. This analysis indicates that replication at 42 degrees C occurs not only from oriV, but also from an origin, oriX, located in the terminal region of the chromosome close to, but distinct from, the prophage rac (oriJ). In an oxa1 mutant of one of these Hfrs, we have shown that replication proceeds at 42 degrees C from all three origins: oriV, oriX, and oriC. Loss of the integrated plasmid results in a temperature- and rich-medium-sensitive strain that replicates the chromosome from oriC and oriX. Replication from oriX proceeds slowly and bidirectionally. We suggest that oriX may be involved in the coupling between replication and cell division.