Nutrient limitaton is a critical signal in Salmonela virulence gene regulation. The katF (rpoS) gene mediates the expression of the Salkionella spy p i virulence genes during bacterial starvation. A katF Salmonella mutant has Increased su bilit to nutrient deprivation, oxidative stress, acid stress, and DNA damage, conditions which are relevant to the intraphagnal environment of host macrophages. Moreover, the katF mutant has g ntly reduced virulence in mice. katF encodes an alternative a factor of RNA polymerase which coordinately regulates Sabnonela viruence.
Fusion plasmids with lacZ under the control of the katE (encoding catalase or hydroperoxidase HPII) and katF (encoding a sigma factor-like protein required for katE expression) promoters were constructed. Expression from both katE and katF promoters was low in rich medium but elevated in poor medium during log-phase growth. Furthermore, the slowdown in growth as cells entered the stationary phase in rich medium, a result of carbon source depletion, was associated with an increase in katE and katF expression. A simple reduction in the carbon source level as the cells entered the stationary phase was not responsible for the increase in expression, because transferring the culture to a medium with no glucose did not induce expression from either promoter. Spent rich medium from stationary-phase cells was capable of inducing expression, as were simple aromatic acids such as benzoate, o-hydroxybenzoate, and p-aminobenzoate added to new medium. Anaerobiosis did not cause an increase in expression, nor did it significantly change the pattern of expression. Regardless of the medium, katF expression was always turned on before or coincidently with katE expression; in the presence of benzoate katF was fully induced, whereas katE was only partially induced, suggesting that a factor in addition to KatF protein was involved in katE expression. During prolonged aerobic incubation, cells lacking katF died off more rapidly than did cells lacking either katE or katG.
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