Evidence for an active role of the DnaK chaperone system in the degradation of σ 32

Abstract

Under non-stressed conditions in Escherichia coli, the heat shock transcription factor σ 32 is rapidly degraded by the AAA protease FtsH. The DnaK chaperone system is also required for the rapid turnover of σ 32 in the cell. It has been hypothesized that the DnaK chaperone system facilitates the degradation of σ 32 by sequestering it from RNA polymerase core. This hypothesis predicts that mutant σ 32 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 32 proteins. Results indicated that the mutant σ 32 proteins as similar as authentic σ 32 were stabilized in Δ dnaK and Δ dnaJ/Δ cbpA cells. The interaction between σ 32 and DnaK/DnaJ/GrpE was not affected by these mutations. These results strongly suggest that the degradation of σ 32 requires an unidentified active role of the DnaK chaperone system.