An RpoH-like heat shock sigma factor is involved in stress response and virulence in Brucella melitensis 16M

B. melitensis 16M genome analysis revealed the presence of six putative sigma factor-encoding genes: rpoD, rpoH1, rpoH2, rpoE1, rpoE2, and rpoN. We mutated all these genes except rpoD. Phenotypic analysis of the mutants reveals that a strain carrying an rpoH2 null mutation (Delta rpoH2) is impaired for growth at 21 and 42 degrees C and shows increased sensitivity to hydrogen peroxide. Compared to the wild-type strain, the Delta rpoH2 mutant is attenuated in all virulence models tested. Three other null mutants (Delta rpoH1, Delta rpoE1, and Delta rpoE2 mutants) are also defective for survival in mice at 4 weeks postinfection. We also demonstrated that rpoH2 deletion strongly reduces the expression of two major virulence factors in B. melitensis, the type IV secretion system and the flagellum.