dif -XerCD is Required for Chromosome Segregation During cSDR-Dependent Growth in Escherichia coli

Escherichia coli cells can grow in a DnaA- and oriC-independent manner in rnhA-deficient cells that lack the major RNaseH activity. This mode of replication is named cSDR (constitutive stable DNA replication) and is believed to initiate from the sites of RNA-DNA hybrids, although the detailed mechanisms of cSDR are unclear. In this study, we discovered that dif sequence and XerC/D are essential for cSDR-dependent growth. cSDR was observed in either dif or xerC/D mutant cells as efficiently as in the control cells, as measured by the incorporation of [3H]-thymine. However, we found that these mutants accumulate cells with extra DNA contents in the presence of rifampicin and cefalexin, suggesting accumulation of unresolved dimeric or multimeric chromosomes or that of catenated chromosomes due to potential problems in resolution of the replicated chromosomes or in decatenation process. These results indicate that site-specific recombination mediated by dif-XerC/D is essential for cSDR-dependent growth.