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Journal of Bacteriology, August 2008, p. 5368-5381, Vol. 190, No. 15
0021-9193/08/$08.00+0     doi:10.1128/JB.00044-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Modes of Overinitiation, dnaA Gene Expression, and Inhibition of Cell Division in a Novel Cold-Sensitive hda Mutant of Escherichia coli

Kazuyuki Fujimitsu, Masayuki Su'etsugu,^, Yoko Yamaguchi,^, Kensaku Mazda,^¶ Nisi Fu, Hironori Kawakami,^|| and Tsutomu Katayama^*

Department of Molecular Biology, Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan

Received 10 January 2008/ Accepted 16 May 2008

The chromosomal replication cycle is strictly coordinated with cell cycle progression in Escherichia coli. ATP-DnaA initiates replication, leading to loading of the DNA polymerase III holoenzyme. The DNA-loaded form of the β clamp subunit of the polymerase binds the Hda protein, which promotes ATP-DnaA hydrolysis, yielding inactive ADP-DnaA. This regulation is required to repress overinitiation. In this study, we have isolated a novel cold-sensitive hda mutant, the hda-185 mutant. The hda-185 mutant caused overinitiation of chromosomal replication at 25°C, which most likely led to blockage of replication fork progress. Consistently, the inhibition of colony formation at 25°C was suppressed by disruption of the diaA gene, an initiation stimulator. Disruption of the seqA gene, an initiation inhibitor, showed synthetic lethality with hda-185 even at 42°C. The cellular ATP-DnaA level was increased in an hda-185-dependent manner. The cellular concentrations of DnaA protein and dnaA mRNA were comparable at 25°C to those in a wild-type hda strain. We also found that multiple copies of the ribonucleotide reductase genes (nrdAB or nrdEF) or dnaB gene repressed overinitiation. The cellular levels of dATP and dCTP were elevated in cells bearing multiple copies of nrdAB. The catalytic site within NrdA was required for multicopy suppression, suggesting the importance of an active form of NrdA or elevated levels of deoxyribonucleotides in inhibition of overinitiation in the hda-185 cells. Cell division in the hda-185 mutant was inhibited at 25°C in a LexA regulon-independent manner, suggesting that overinitiation in the hda-185 mutant induced a unique division inhibition pathway.

Published ahead of print on 23 May 2008.

These authors contributed equally to this study.

Present address: Institute for Cell and Molecular Biosciences, Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne, United Kingdom.

Present address: Shionogi & Co., Ltd., Osaka, Japan.

^¶ Present address: Nipro Co., Osaka, Japan.

^|| Present address: Cold Spring Harbor Laboratory, Cold Spring Harbor, NY.