Twocomponent Systems and the Regulation of Virulence Gene Expression

The signal transduction mechanisms allowing bacteria to modulate gene expression in response to diverse stimuli often involve two-component systems which are composed of a sensor kinase and a response regulator. TCS have not been studied intensively in L. monocytogenes: lisRK [94], cheYA [95], agrAC [96], and cesRK [97] were characterized in some detail and shown to contribute to virulence of L. monocytogenes. The availability of the complete genomic sequence of L. monocytogenes EGD-e allowed the in silico identification of 16 putative two-component systems. Surprisingly, only one out these 16 had no counterpart in the closely related, but apathogenic species L. innocua [2]. The role of the listerial two-component systems for in vitro and in vivo survival and growth was studied systematically by the construction of in-frame deletion mutants in the 15 nonessential two-component systems [98]. Careful analysis of the mutants revealed that the previously uncharacterized response regulator DegU (encoded by lmo2515) contributes to virulence and is necessary for motility since DegU regulates flagellar gene expression [98, 99]. Interestingly, DegU is the only response regulator identified in the genome sequence which obviously lacks a cognate histidine kinase.

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