Journal of Bacteriology, September 2001, p. 5302-5310, Vol. 183, No. 18
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.18.5302-5310.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
32 Homolog (RpoH) Plays a Major Role in the Heat Shock
Response of Agrobacterium tumefaciens

andHSP Research Institute, Kyoto Research Park, Kyoto 600-8813, Japan
Received 26 February 2001/Accepted 19 June 2001
RpoH (Escherichia coli
32 and
its homologs) is the central regulator of the heat shock response in
gram-negative proteobacteria. Here we studied salient regulatory
features of RpoH in Agrobacterium tumefaciens by
examining its synthesis, stability, and activity while increasing the
temperature from 25 to 37°C. Heat induction of RpoH synthesis
occurred at the level of transcription from an RpoH-dependent promoter,
coordinately with that of DnaK, and followed by an increase in the RpoH
level. Essentially normal induction of heat shock proteins was observed
even with a strain that was unable to increase the RpoH level upon heat
shock. Moreover, heat-induced accumulation of dnaK mRNA
occurred without protein synthesis, showing that preexisting RpoH was
sufficient for induction of the heat shock response. These results
suggested that controlling the activity, rather than the amount, of
RpoH plays a major role in regulation of the heat shock response. In
addition, increasing or decreasing the DnaK-DnaJ chaperones
specifically reduced or enhanced the RpoH activity, respectively. On
the other hand, the RpoH protein was normally stable and remained
stable during the induction phase but was destabilized transiently
during the adaptation phase. We propose that the DnaK-mediated
control of RpoH activity plays a primary role in the induction of heat
shock response in A. tumefaciens, in contrast to what
has been found in E. coli.
Present address: Department of Biophysics, Graduate School of
Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
Present address: Sumitomo Pharmaceutical Co., Niihama
792-0001, Japan.
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