Conversion to bidirectional replication after unidirectional initiation from R1 plasmid origin integrated at oriC in Escherichia coli

The cell division phenotypes of Escherichia coli with its chromosome replic ation driven by oriR (from plasmid R1) were examined by fluorescence micros copy and flow cytometry. Chromosome replication patterns in these strains w ere followed by marker frequency analyses. In one of the strains, the unidi rectional oriR was integrated so that the replication fork moved clockwise from the oriC region, and bacterial growth and division were similar to tho se of the wild-type parent. The bacteria were able to convert the unidirect ional initiation from oriR into bidirectional replication. The site for con version of uni- to bidirectional replication seemed to be localized and cou ld be mapped genetically within 6 min to the immediate right of the minimal oriC. Replication starting in the counterclockwise direction from the R1 r eplicon integrated at the same site in the opposite orientation could not b e described as either bi- or unidirectional, as no single predominant origi n could be discerned from the more or less flat marker frequency pattern. T hese strains also showed extensive filamentation, irregular nucleoid distri bution and the presence of anucleate cells, indicative of segregation and d ivision defects. Comparison among intR1 derivatives differing in the positi on of the integrated oriR relative to the chromosome origin suggested that the oriC sequence itself was dispensable for the conversion to bidirectiona lity. However, passage of the replication fork over the 6 min region to the right of oriC seemed important for the bidirectional replication pattern a nd normal cell division phenotype.