Under non-stressed conditions in Escherichia coli, the heat shock transcription factor σ 3 2 is rapidly degraded by the AAA protease FtsH. The DnaK chaperone system is also required for the rapid turnover of σ 3 2 in the cell. It has been hypothesized that the DnaK chaperone system facilitates the degradation of σ 3 2 by sequestering it from RNA polymerase core. This hypothesis predicts that mutant σ 3 2 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 σ 3 2 proteins. Results indicated that the mutant σ 3 2 proteins as similar as authentic σ 3 2 were stabilized in ΔdnaK and ΔdnaJ/ΔcbpA cells. The interaction between σ 3 2 and DnaK/DnaJ/GrpE was not affected by these mutations. These results strongly suggest that the degradation of σ 3 2 requires an unidentified active role of the DnaK chaperone system.