Insertion of an R1 plasmid into the origin of replication of the E. coli chromosome: random timing of replication of the hybrid chromosome.

A 16 bp BgI II fragment was deleted in vitro from the minimal origin of replication of the Escherichia coli chromosome, oriC, and was replaced by a 10 kb R1 miniplasmid, pKN1562, containing the basic R1 replicon and a kanamycin resistance gene. The deletion-insertion was transferred by homologous recombination into the chromosome of a dnaA(ts) strain. P1 transduction separated the origin "mutation" from the dnaA46 allele. Integration of mini-R1 into oriC was verified by Southern blotting and by analysis of the R1 incompatibility phenotype. It was possible to isolate normal R1 miniplasmids from the integrated R1. Chromosome replication was initiated at random times after a short delay. The constructed strains grew 20%-30% slower than the wild type and showed more heterogeneous cell sizes.