Abstract
Bis-(3’,5’)-cyclic diguanylic acid, also known as cyclic-di-GMP (CdG) is a secondary messenger that plays important roles in the basic physiology and virulence of many bacteria. CdG functions in three ways, by binding to effector proteins, binding to transcriptional regulators, or binding to riboswitches and modulating their activity. The latter two effect gene transcription while the former modulates protein function. Diguanylate cyclases (DGCs) produce CdG from two GTP molecules, while phosphodiesterases (PDE) degrade it. Their regulation affects the level and activity of CdG with in the cell. Previous work demonstrated a link between CdG signaling and Brucella virulence, but the genetic regulators that mediate this link have not been identified. The recent discovery and characterization of the CdG- responsive UDP-xylose regulator (CuxR) in Sinorhizobium meliloti, a close phylogenetic relative of Brucella, has provided a possible clue in this regard. The DNA-binding activity of CuxR, an AraC-type transcriptional activator, is dependent on the presence of CdG. CuxR controls the expression of genes encoding an arabinose-based exopolysaccharide known as APS in S. meliloti in a CdG-responsive manner. The genes encoding CuxR and the aps operon reside in a single genetic locus inS. meliloti that is conserved in Brucella. Derivatives of virulent strain B. abortus 2308 with null mutations in the cuxR and aps genes are currently being constructed to determine if - a) CuxR mediates CdG signaling in Brucella; b) if CuxR regulates expression of the Brucella aps genes, and c) if the cuxR and/or aps genes are required for the wild-type virulence of B. abortus 2308 in mice and cultured mammalian cells. In addition to providing important insight into the specific role that CdG signaling plays in Brucella virulence, these studies also have the potential to define a link between exopolysaccharide (EPS) production and virulence in these bacteria. This is an exciting possibility because although Brucella strains have been shown to have the genetic capacity to produce EPSs, it is unknown if they play a role in the pathogenesis of brucellae in their mammalian hosts.
