A 2-COMPONENT REGULATORY SYSTEM PLAYING A CRITICAL ROLE IN PLANT-PATHOGENS AND ENDOSYMBIONTS IS PRESENT IN BRUCELLA-ABORTUS AND CONTROLS CELL INVASION AND VIRULENCE

Two mutants showing increased sensitivity to polycations and surfactan ts were obtained by transposon mutagenesis of virulent Brucella abortu s 2308 Nal(r). These mutants showed no obvious in vitro growth defects and produced smooth-type lipopolysaccharides. However, they hardly mu ltiplied or persisted in mouse spleens, displayed reduced invasiveness in macrophages and HeLa cells, lost the ability to inhibit lysosome f usion and were unable to replicate intracellularly. Subsequent DNA ana lyses identified a two-component regulatory system [Brucella virulence related (Bvr)] with a regulatory (BvrR) and sensory (BvrS) protein. C loning of bvrR in the BvrR-deficient mutant restored the resistance to polycations and, in part, the invasiveness and the ability to multipl y intracellularly. BvrR and BvrS were highly similar (87-89% and 70-80 % respectively) to the regulatory and sensory proteins of the chromoso mally encoded Rhizobium meliloti Chvl-ExoS and Agrobacterium tumefacie ns Chvl-ChvG systems previously shown to be critical for endosymbiosis and pathogenicity in plants. Divergence among the three sensory prote ins was located mostly within a periplasmic domain probably involved i n stimulus sensing. As B. abortus, R. meliloti and A. tumefaciens are phylogenetically related, these observations suggest that these system s have a common ancestor that has evolved to sense stimuli in plant an d animal microbial environments.