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Region 2.1 of the Escherichia coli heat-shock sigma factor RpoH (³²) is necessary but not sufficient for degradation by the FtsH protease

¹ Institute of Microbiology, ETH Zürich, Switzerland
² Institute of Microbial Biology, Ruhr University Bochum, D-44780 Bochum, Germany
³ Institute of Microbiology, Free University Berlin, Germany

Correspondence
Franz Narberhaus
franz.narberhaus{at}rub.de

The cellular level of the Escherichia coli heat-shock sigma factor RpoH (³²) is negatively controlled by chaperone-mediated proteolysis through the essential metalloprotease FtsH. Point mutations in the highly conserved region 2.1 stabilize RpoH in vivo. To assess the importance of this turnover element, hybrid proteins were constructed between E. coli RpoH and Bradyrhizobium japonicum RpoH₁, a stable RpoH protein that differs from region 2.1 of E. coli RpoH at several positions. Nine amino acids forming a putative -helix were exchanged between the two proteins. Both hybrids were active sigma factors and showed intermediate protein stability. Introduction of RpoH region 2.1 into the general stress sigma factor RpoS, which is a substrate of the ClpXP protease, did not render RpoS susceptible to FtsH. Hence, region 2.1 alone is not sufficient to confer FtsH sensitivity to other proteins. Region 2.1 is not a major chaperone-binding site since DnaK and DnaJ bound efficiently to all RpoH variants. The in vivo stability of the mutated RpoH proteins correlated with their stability in a purified in vitro degradation system, suggesting that region 2.1 might be directly involved in the interaction with the FtsH protease.

This work is dedicated to the memory of Amos Oppenheim, who was involved in the initial stages of this project.

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