Xanthomonas campestris cell-cell communication involves a putative nucleotide receptor protein Clp and a hierarchical signalling network

The bacterial pathogen Xanthomonas campestris pv. campestris co-ordinates virulence factor production and biofilm dispersal through a diffusible signal factor (DSF)-mediated cell-cell communication mechanism. The RpfC/RpfG two-component system plays a key role in DSF signal transduction and appears to modulate downstream DSF regulon by changing intracellular content of cyclic dimeric GMP (c-di-GMP), an unusual nucleotide second messenger. Here we show that Clp, a conserved global regulator showing a strong homology to the cAMP nucleotide receptor protein Crp of Escherichia coli, is essential for DSF regulation of virulence factor production but not for biofilm dispersal. Deletion of clp in Xcc changed the transcriptional expression of 299 genes including a few encoding transcription factors. Further genetic and microarray analysis led to identification of a homologue of the transcriptional regulator Zur, and a novel TetR-type transcription factor FhrR. These two regulatory factors regulated different sets of genes within Clp regulon. These results outline a hierarchical signalling network by which DSF modulates different biological functions, and may also provide a clue on how the novel nucleotide signal can be coupled to its downstream regulatory networks.