ABSTRACT
The α-2 Proteobacteria include pathogenic intracellular and extracellular bacteria of animals and plants as well as plant endosymbionts sharing conserved two-component regulatory systems. Brucella BvrS/BvrR is similar to the ChvG(ExoS)/ Chvl of Agrobacterium and Sinorhizobium, and to the BatS/BatR putative system of Bartonella. Moreover, the structure around the genes encoding these systems is essentially the same. Dysfunction of these systems alters the cell envelopes: bvrS/bvrR mutants display increased surface hydrophobicity, sensitivity to bactericidal peptides and detergents, and altered lipid A acylation; chvG mutants are sensitive to detergents and acidic pH. Moreover, BvrS/BvrR regulates transcription of at least Omp25 and Omp22, ChvG/ChvI regulates the acidic pH-inducible outer membrane protein Aop (and also genes katA, virB and virE), and S. meliloti ExoS regulates the production of succinoglycan. These systems are critical in the interaction with eukaryotic cells: bvrS and bvrR mutants are avirulent in mice, sensitive to serum, and hampered in cell penetration and intracellular trafficking; chvG and chvl mutants lack tumour inducing ability; and S. meliloti succinoglycan-deficient mutants cannot establish symbiosis. These similarities suggest that these systems have a common ancestor that, while evolving to sense pericellular or intracellular signals, has kept the control of cell surface characteristics. Moreover, they suggest a co-evolution of the systems and the cognate surface molecules.
