Heat shock regulation in the ftsH null mutant of Escherichia coli:: dissection of stability and activity control mechanisms of σ32 in vivo

The heat shock response of Escherichia coli is regulated by the cellular level and the activity of sigma(32), a, alternative sigma factor for heat shock promoters. FtsH, a membrane-bound AAA-type metalloprotease, degrades sigma(32) and has a central role in the control of the sigma(32) level. The ftsH null mutant was isolated, and establishment of the Delta ftsH mutant allowed us to investigate control mechanisms of the stability and the activity of sigma(32) separately in vivo. Loss of the FtsH function caused marked stabilization and consequent accumulation of sigma(32) (approximate to 20-fold Of the wild type), leading to the impaired downregulation of the level of sigma(32). Surprisingly, however, Delta ftsH cells express heat shock proteins only two- to threefold higher than wild-type cells, and they also show almost normal heat shock response upon temperature upshift, These results indicate the presence of a control mechanism that downregulates the activity of sigma(32) when it is accumulated. Overproduction of DnaK/J reduces the activity of sigma(32) in Delta ftsH cells without any detectable changes in the level of sigma(32), indicating that the DnaK chaperone system is responsible for the activity control of sigma(32) in vivo. In addition, CbpA, an analogue of DnaJ, was demonstrated to have overlapping functions with DnaJ in both the activity and the stability control of sigma(32).