Fig 8. Model for Chr2 replication licensing by remodeled RctB.

The triggering of Chr2 replication initiation requires two copies of crtS. The two copies are normally produced by passage of the Chr1 replication fork across the crtS site (left diagram) or, can also be provided in the absence of Chr1 replication by inserting an extra copy of crtS (right diagram). This suggests that the product of replication but not the passage of the fork per se is obligatory for crtS function. In our experiments, the requirement for a second copy of crtS could be obviated by increasing RctB supply (Fig 4). This, together with the findings that to date all revertants of ΔcrtS map in rctB and that crtS alters the DNA binding activities of RctB, lead us to propose that crtS activates RctB. The two crtS copies are apparently required to activate RctB sufficiently for initiation.