A reduction in ribonucleotide reductase activity slows down the chromosome replication fork but does not change its localization.
BACKGROUND
It has been proposed that the enzymes of nucleotide biosynthesis may be compartmentalized or concentrated in a structure affecting the organization of newly replicated DNA. Here we have investigated the effect of changes in ribonucleotide reductase (RNR) activity on chromosome replication and organization of replication forks in Escherichia coli.


RESULTS
Reduced concentrations of deoxyribonucleotides (dNTPs) obtained by reducing the activity of wild type RNR by treatment with hydroxyurea or by mutation, resulted in a lengthening of the replication period. The replication fork speed was found to be gradually reduced proportionately to moderate reductions in nucleotide availability. Cells with highly extended C periods showed a "delay" in cell division i.e. had a higher cell mass. Visualization of SeqA structures by immunofluorescence indicated no change in organization of the new DNA upon moderate limitation of RNR activity. Severe nucleotide limitation led to replication fork stalling and reversal. Well defined SeqA structures were not found in situations of extensive replication fork repair. In cells with stalled forks obtained by UV irradiation, considerable DNA compaction was observed, possibly indicating a reorganization of the DNA into a "repair structure" during the initial phase of the SOS response.


CONCLUSIONS
The results indicate that the replication fork is slowed down in a controlled manner during moderate nucleotide depletion and that a change in the activity of RNR does not lead to a change in the organization of newly replicated DNA. Control of cell division but not control of initiation was affected by the changes in replication elongation.
Odsbu
Ingvild
I
Department of Cell Biology, Institute for Cancer Research, The Norwegian Radium Hospital, Oslo University Hospital, Oslo, Norway.
Morigen
Skarstad
Kirsten
K
D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000502:physiology*; D015139:Blotting, Southern; D002453:Cell Cycle; D002455:Cell Division; D002875:Chromosomes / Q000648:ultrastructure*; D004247:DNA / Q000378:metabolism; D004261:DNA Replication; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D029968:Escherichia coli Proteins / Q000235:genetics / Q000502:physiology*; D005434:Flow Cytometry / Q000379:methods; D006918:Hydroxyurea / Q000378:metabolism; D008957:Models, Genetic; D009154:Mutation; D010641:Phenotype; D012264:Ribonucleotide Reductases / Q000378:metabolism*; D014466:Ultraviolet Rays
D001425:Bacterial Outer Membrane Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; C089746:SeqA protein, E coli; D004247:DNA; D012264:Ribonucleotide Reductases; D006918:Hydroxyurea