Escherichia coli strains in which chromosome replication is controlled by a P1 or F replicon integrated into oriC

We report the construction of intP1 and intFs strains, in which the basic replicon from either plasmid pi or plasmid F (oriS) has been integrated in both orientations into the origin of replication, oriC, of the Escherichia coli chromosome. In these strains, oriC is no longer functional and chromosome-replication is instead controlled by the integrated plasmid replicon. The strains were viable, showing that the deviation from normal chromosome-replication control was not large enough to prohibit cell survival. The strains showed a broader cell-size distribution than a wild-type strain and were more filamentous in rich than in minimal media, although cells of wild-type size were also present. Cells which contained aberrantly shaped or aberrantly distributed nucleoids were also observed. Marker-frequency analysis indicated that chromosome replication was predominantly bidirectional in both intFs strains. In the intP1 strains, the degree of bidirectionality depended upon the orientation of the integrated replicon.