Missense mutations in the 3' end of the Escherichia coli dnaG gene do not abolish primase activity but do confer the chromosome-segregation-defective (par) phenotype

Isogenic dnaG strains of Escherichia coli with the parB and dnaC2903 alleles in the MG1655 chromosomal background displayed the classic par phenotype at the nonpermissive temperature of 42 degrees C. These strains synthesized DNA at 42 degrees C, but remained chromosome segregation defective as determined by cytology. A strain with the dnaG2903 allele was tested for its ability to support DNA replication of a primase-dependent G4ori(c)-containing M13 phage derivative by quantitative competitive PCR (QC-PCR). The dnaG2903 strain converted the single-stranded DNA into double-stranded replicative form DNA at 42 OC. These results indicate that DnaG2903 retains primase activity at the restrictive temperature. Nucleoids remained unsegregated in the central region of cell filaments at 42 OC. The observed suppression of cell filamentation in dnaG sfiA or dnaG lexA double mutants suggests that the SOS response is induced at the restrictive temperature in parB and dnaC2903 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.