title: Reduced initiation frequency from oriC restores viability of a temperature-sensitive Escherichia coli replisome mutant.
abstract: The dnaX gene of Escherichia coli encodes tau and gamma clamp loader subunits of the replisome. Cells carrying the temperature-sensitive dnaX2016 mutation were induced for the SOS response at non-permissive temperature. The SOS induction most likely resulted from extensive replication fork collapse that exceeded the cells' capacity for restart. Seven mutations in the dnaA gene that partly suppressed the dnaX2016 temperature sensitivity were isolated and characterized. Each of the mutations caused a single amino acid change in domains III and IV of the DnaA protein, where nucleotide binding and DNA binding, respectively, reside. The diversity of dnaA(Sx) mutants obtained indicated that a direct interaction between the DnaA protein and tau or gamma is unlikely and that the mechanism behind suppression is related to DnaA function. All dnaA(Sx) mutant cells were compromised for initiation of DNA replication, and contained fewer active replication forks than their wild-type counterparts. Conceivably, this led to a reduced number of replication fork collapses within each dnaX2016 dnaA(Sx) cell and prevented the SOS response. Lowered availability of wild-type DnaA protein also led to partial suppression of the dnaX2016 mutation, confirming that the dnaA(Sx) mode of suppression is indirect and results from a reduced initiation frequency at oriC.
authors.affiliation: Department of Life Sciences and Chemistry, 18-1, Roskilde University, PO Box 260, DK-4000 Roskilde, Denmark.
authors.forename: Ole
authors.initials: O
authors.lastname: Skovgaard
authors.affiliation: Department of Life Sciences and Chemistry, 18-1, Roskilde University, PO Box 260, DK-4000 Roskilde, Denmark.
authors.forename: Anders
authors.initials: A
authors.lastname: Løbner-Olesen
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D004258:DNA Polymerase III / Q000235:genetics / Q000378:metabolism; D004261:DNA Replication; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D016153:Genes, Suppressor; D009154:Mutation; D018741:Replication Origin / Q000235:genetics*; D013014:SOS Response, Genetics; D013696:Temperature
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; C066312:DnaA protein, Bacteria; C103076:DnaX protein, Bacteria; D029968:Escherichia coli Proteins; D004258:DNA Polymerase III