A homologue of an operon required for DNA transfer in Agrobacterium is required in Brucella abortus for virulence and intracellular multiplication

As part of a Brucella abortus 2308 genome project carried out in our labora tory, we identified, cloned, and sequenced a genomic DNA fragment containin g a locus (virB) highly homologous to bacterial type IV secretion systems. The B. abortus virB Locus is a collinear arrangement of 13 open reading fra mes (ORFs), Between virB1 and virB2 and downstream of ORF12, two degenerate d, palindromic repeat sequences characteristic of Brucella intergenic regio ns were found. Gene reporter studies demonstrated that the B. abortus virB locus constitutes an operon transcribed from virB1 which is turned on durin g the stationary phase of growth. A B. abortus polar virB1 mutant failed to replicate in HeLa cells, indicating that the virB operon plays a critical role in intracellular multiplication. Mutants with polar and nonpolar mutat ions introduced in virB10 showed different behaviors in mice and in the HeL a cell infection assay, suggesting that virB10 per se is necessary for the correct function of this type TV secretion apparatus. Mouse infection assay s demonstrated that the virB operon constitutes a major determinant of B. a bortus virulence. It is suggested that putative effector molecules secreted by this type TV secretion system determine routing of B, abortus to an end oplasmic reticulum-related replication compartment.