Marked instability of the sigma(32) heat shock transcription factor at high temperature. Implications for heat shock regulation.

The heat shock response in Escherichia coli depends on a transient increase in the intracellular level of sigma(32) that results from both increased synthesis and transient stabilization of normally unstable sigma(32). Although the membrane-bound ATP-dependent protease FtsH (HflB) plays an important role in degradation of sigma(32), our previous results suggested that several cytosolic ATP-dependent proteases including HslVU (ClpQY) are also involved in sigma(32) degradation (Kanemori, M., Nishihara, K., Yanagi, H., and Yura, T. (1997) J. Bacteriol. 179, 7219-7225). We now report on the ATP-dependent proteolysis of sigma(32) by purified HslVU protease and its unusual dependence on high temperature: sigma(32) was rapidly degraded at 44 degrees C, but with much slower rates ( approximately 15-fold) at 35 degrees C. FtsH-dependent degradation of sigma(32) also gave similar results. In agreement with these results in vitro, the turnover of sigma(32) in normally growing cells at high temperature (42 degrees C) was much faster than at low temperature (30 degrees C). Taken together with other evidence, these results suggest that the sigma(32) level during normal growth is primarily determined by the stability (susceptibility to proteases) and synthesis rate of sigma(32) set by ambient temperature, whereas fine adjustment such as transient stabilization of sigma(32) observed upon heat shock is brought about through monitoring changes in the cellular state of protein folding.