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Journal of Bacteriology, December 1999, p. 7509-7515, Vol. 181, No. 24
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Differential and Independent Roles of a sigma 32 Homolog (RpoH) and an HrcA Repressor in the Heat Shock Response of Agrobacterium tumefaciens

Kenji Nakahigashi,1,dagger Eliora Z. Ron,2 Hideki Yanagi,1 and Takashi Yura1,*

HSP Research Institute, Kyoto Research Park, Shimogyo-ku, Kyoto 600-8813, Japan,1 and Department of Molecular Microbiology and Biotechnology, The George S. Wise Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv, Israel2

Received 12 July 1999/Accepted 6 October 1999

The heat shock response in alpha proteobacteria is unique in that a combination of two regulators is involved: a positive regulator, RpoH (sigma 32 homolog), found in the alpha, beta, and gamma proteobacteria, and a negative regulator, HrcA, widely distributed in eubacteria but not in the gamma proteobacteria. To assess the differential roles of the two regulators in these bacteria, we cloned the hrcA-grpE operon of Agrobacterium tumefaciens, analyzed its transcription, and constructed deletion mutants lacking RpoH and/or HrcA. The Delta rpoH mutant and Delta rpoH Delta hrcA double mutant were unable to grow above 30°C. Whereas the synthesis of heat shock proteins (e.g., DnaK, GroEL, and ClpB) was transiently induced upon temperature upshift from 25 to 37°C in the wild type, such induction was not observed in the Delta rpoH mutant, except that GroEL synthesis was still partially induced. By contrast, the Delta hrcA mutant grew normally and exhibited essentially normal heat induction except for a higher level of GroEL expression, especially before heat shock. The Delta rpoH Delta hrcA double mutant showed the combined phenotypes of each of the single mutants. The amounts of dnaK and groE transcripts before and after heat shock, as determined by primer extension, were consistent with those of the proteins synthesized. The cellular level of RpoH but not HrcA increased significantly upon heat shock. We conclude that RpoH plays a major and global role in the induction of most heat shock proteins, whereas HrcA plays a restricted role in repressing groE expression under nonstress conditions.

* Corresponding author. Mailing address: HSP Research Institute, Kyoto Research Park, Shimogyo-ku, Kyoto 600-8813, Japan. Phone: 81-75-315-8619. Fax: 81-75-315-8659. E-mail: tyura{at}hsp.co.jp.

dagger Present address: Department of Biophysics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.

Journal of Bacteriology, December 1999, p. 7509-7515, Vol. 181, No. 24
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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