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 ba sic replicon from either plasmid pi or plasmid F (oriS) has been integ rated in both orientations into the origin of replication, oriC, of th e Escherichia coil chromosome. In these strains, oriC is no longer fun ctional and chromosome-replication is instead controlled by the integr ated plasmid replicon. The strains were viable, showing that the devia tion from normal chromosome-replication control was not large enough t o prohibit cell survival. The strains showed a broader cell-size distr ibution 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 nuc leoids were also observed. Marker-frequency analysis indicated that ch romosome replication was predominantly bidirectional in both intFs str ains. In the intP1 strains, the degree of bidirectionality depended up on the orientation of the integrated replicon.