Marked Instability of the ς32 Heat Shock Transcription Factor at High Temperature

Abstract

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