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Journal of Bacteriology, April 2002, p. 1843-1850, Vol. 184, No. 7
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.7.1843-1850.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Effect of Different Concentrations of H-NS Protein on Chromosome Replication and the Cell Cycle in Escherichia coli

T. Atlung1* and F. G. Hansen2

Department of Life Sciences and Chemistry, Roskilde University, DK-4000 Roskilde,1 Section of Molecular Microbiology, BioCentrum-DTU, Technical University of Denmark, DK-2800 Lyngby, Denmark2

Received 10 July 2001/ Accepted 2 January 2002

Flow cytometric analysis showed that the hns205 and hns206 mutants, lacking the abundant nucleoid-associated protein H-NS, have decreased origin concentration, as well as a low number of origins per cell (ploidy). The most striking observation was that the low ploidy was due to a very short replication time, e.g., at 30°C it was halved compared to that of the hns+ strain. The decreased origin concentration was not caused by a decreased dnaA gene expression, and the hns206 mutant had normal DnaA protein concentrations. The replication phenotypes of the hns206 mutant were independent of RpoS. Cells overproducing H-NS from a LacI-controlled plasmid had a normal origin concentration, indicating that H-NS is not controlling initiation. A wild-type H-NS concentration is, however, required to obtain a wild-type origin concentration, since cells with an intermediate H-NS concentration had an intermediate origin concentration. Two lines of evidence point to an indirect effect of H-NS on initiation. First, H-NS did not show high-affinity binding to any part of oriC, and H-NS had no effect on transcription entering oriC from the mioC promoter. Second, in a shift experiment with the hns206 mutant, when H-NS protein was induced to wild-type levels within 10 min, it took more than one generation before the origin concentration started to increase.

* Corresponding author. Mailing address: Department of Life Sciences and Chemistry, Roskilde University, DK-4000 Roskilde, Denmark. Phone: (45) 46-74-24-02. Fax: (45) 46-74-30-11. E-mail: atlung{at}ruc.dk.

Journal of Bacteriology, April 2002, p. 1843-1850, Vol. 184, No. 7
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.7.1843-1850.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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