Isogenic strains of with the and alleles in the MG1655 chromosomal background displayed the classic phenotype at the nonpermissive temperature of 42 °. These strains synthesized DNA at 42 °, but remained chromosome segregation defective as determined by cytology. A strain with the allele was tested for its ability to support DNA replication of a primase-dependent G4ori-containing M13 phage derivative by quantitative competitive PCR (QC-PCR). The strain converted the single-stranded DNA into double-stranded replicative form DNA at 42 °. These results indicate that DnaG2903 retains primase activity at the restrictive temperature. Nucleoids remained unsegregated in the central region of cell filaments at 42 °. The observed suppression of cell filamentation in or double mutants suggests that the SOS response is induced at the restrictive temperature in and strains but fails to account entirely for the cell filamentation phenotype. ParB and DnaG2903 presumably can synthesize primer RNA for DNA replication, but may be defective in their interactions with DNA replication proteins, cell cycle regulatory factors, or the chromosome segregation apparatus itself.


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