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Toh-ru Shimuta, Kiyotaka Nakano^,a, Yoko Yamaguchi, Shogo Ozaki, Kazuyuki Fujimitsu, Chika Matsunaga, Kenji Noguchi^b, Akiko Emoto^c and Tsutomu Katayama^*

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

Escherichia coli DnaA protein initiates chromosomal replication and is an important regulatory target during the replication cycle. In this study, a suppressor mutation isolated by transposon mutagenesis was found to allow growth of the temperature-sensitive dnaA508 and dnaA167 mutants at 40 °C. The suppressor consists of a transposon insertion in a previously annotated ORF, here termed hspQ, a novel heat shock gene whose promoter is recognized by the major heat shock sigma factor ³². Expression of hspQ on a pBR322 derivative inhibits growth of the dnaA508 and dnaA167 mutants at 30 °C, whereas growth of dnaA46 and other dnaA mutants is insensitive to changes in the level of hspQ. Cellular DnaA508 protein is degraded rapidly at elevated temperature, but hspQ disruption impedes this process. In contrast, DnaA46 protein is rapidly degraded in an hspQ-independent manner. Gel-filtration and chemical cross-linking experiments suggest that HspQ forms a stable homodimer in solution and can form homomultimers consisting of about four monomers. Heat-shock induced proteases such as Clp contain homomultimers of subunit proteins. We propose that HspQ is a new factor involved in the quality control of proteins and that it functions by excluding denatured proteins.

Present addresses: ^aChugai Pharmaceutical Co., Tokyo, Japan;

^bKaketsuken Institute, Kumamoto, Japan;

^cDepartment of Pharmacy, Saga University Hospital, Saga, Japan

These authors contributed equally to this work.

^* Correspondence: E-mail: katayama{at}phar.kyushu-u.ac.jp

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