Under non-stressed conditions in Escherichia coli, the heat shock transcription factor σ32 is rapidly degraded by the AAA protease FtsH. The DnaK chaperone system is also required for the rapid turnover of σ32 in the cell. It has been hypothesized that the DnaK chaperone system facilitates the degradation of σ32 by sequestering it from RNA polymerase core. This hypothesis predicts that mutant σ32 proteins, which are deficient in binding to RNA polymerase core, will be degraded independently of the DnaK chaperone system. We examined the in vivo stability of such mutant σ32 proteins. Results indicated that the mutant σ32 proteins as similar as authentic σ32 were stabilized in ΔdnaK and ΔdnaJ/ΔcbpA cells. The interaction between σ32 and DnaK/DnaJ/GrpE was not affected by these mutations. These results strongly suggest that the degradation of σ32 requires an unidentified active role of the DnaK chaperone system.
