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J Bacteriol, May 1998, p. 2402-2408, Vol. 180, No. 9
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Regulatory Conservation and Divergence of sigma 32 Homologs from Gram-Negative Bacteria: Serratia marcescens, Proteus mirabilis, Pseudomonas aeruginosa, and Agrobacterium tumefaciens

Kenji Nakahigashi, Hideki Yanagi, and Takashi Yura*

HSP Research Institute, Kyoto Research Park, Kyoto 600, Japan

Received 10 November 1997/Accepted 27 February 1998

The heat shock response in Escherichia coli is mediated primarily by the rpoH gene, encoding sigma 32, which is specifically required for transcription of heat shock genes. A number of sigma 32 homologs have recently been cloned from gram-negative bacteria that belong to the gamma or alpha subdivisions of the proteobacteria. We report here some of the regulatory features of several such homologs (RpoH) expressed in E. coli as well as in respective cognate bacteria. When expressed in an E. coli Delta rpoH strain lacking its own sigma 32, these homologs activated the transcription of heat shock genes (groE and dnaK) from the start sites normally used in E. coli. The level of RpoH in Serratia marcescens and Pseudomonas aeruginosa cells was very low at 30°C but was elevated markedly upon a shift to 42°C, as found previously with E. coli. The increased RpoH levels upon heat shock resulted from both increased synthesis and stabilization of the normally unstable RpoH protein. In contrast, the RpoH level in Proteus mirabilis was relatively high at 30°C and increased less markedly upon heat shock, mostly by increased synthesis; this sigma 32 homolog was already stable at 30°C, and little further stabilization occurred upon the shift to 42°C. The increased synthesis of RpoH homologs in all these gamma proteobacteria was observed even in the presence of rifampin, suggesting that the induction occurred at the level of translation. Thus, the basic regulatory strategy of the heat shock response by enhancing the RpoH level is well conserved in the gamma proteobacteria, but some divergence in the actual mechanisms used occurred during evolution.

* Corresponding author. Mailing address: HSP Research Institute, Kyoto Research Park, Kyoto 600, Japan. Phone: (81)-75-315-8619. Fax: (81)-75-315-8659. E-mail: tyura{at}hsp.co.jp.

J Bacteriol, May 1998, p. 2402-2408, Vol. 180, No. 9
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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