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The heat shock response of Escherichia coli is regulated by the cellular level and the activity of σ32, an alternative sigma factor for heat shock promoters. FtsH, a membrane-bound AAA-type metalloprotease, degrades σ32 and has a central role in the control of the σ32 level. The ftsH null mutant was isolated, and establishment of the Δ ftsH mutant allowed us to investigate control mechanisms of the stability and the activity of σ32 separately in vivo . Loss of the FtsH function caused marked stabilization and consequent accumulation of σ32 (≈20-fold of the wild type), leading to the impaired downregulation of the level of σ32. Surprisingly, however, Δ 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 σ32 when it is accumulated. Overproduction of DnaK/J reduces the activity of σ32 in Δ ftsH cells without any detectable changes in the level of σ32, indicating that the DnaK chaperone system is responsible for the activity control of σ32 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 σ32.  相似文献   

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