The ClpXP ATP-Dependent Protease Regulates Flagellum Synthesis inSalmonella entericaSerovar Typhimurium

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Abstract
The ClpXP protease is a member of the ATP-dependent protease family and plays a dynamic role in the control of availability of regulatory proteins and the breakdown of abnormal and misfolded proteins. The proteolytic activity is rendered by the ClpP component, while the substrate specificity is determined by the ClpX component that has ATPase activity. We describe here a new role of the ClpXP protease inSalmonella entericaserovar Typhimurium in which ClpXP is involved in the regulation of flagellum synthesis. Cells deleted for ClpXP show “hyperflagellate phenotype,” exhibit overproduction of the flagellar protein, and show a fourfold increase in the rate of transcription of thefliCencoding flagellar filament. The assay for promoter activity of the genes responsible for expression of thefliCshowed that the depletion of ClpXP results in dramatic enhancement of the expression of thefliAencoding sigma factor ς28, leaving the expression level of theflhDmaster operon lying at the top of the transcription hierarchy of flagellar regulon almost normal. These results suggest that the ClpXP may be responsible for repressing the expression of flagellar regulon through the control of the FlhD/FlhC master regulators at the posttranscriptional and/or posttranslational levels. Proteome analysis of proteins secreted from the mutant cells deficient forflhDCandclpXPgenes demonstrated that the ΔflhDmutation abolished the enhanced effect by ΔclpXPmutation on the production of flagellar proteins, suggesting that the ClpXP possibly defines a regulatory pathway affecting the expression of flagellar regulon that is dependent on FlhD/FlhC master regulators.