ExoR, a novel genetic regulator in <italic>Agrobacterium tumefaciens</italic>, influences diverse processes including biofilm formation, cell surface functions, motility and virulence.

Abstract

Agrobacterium tumefaciens is a Gram-negative soil bacterium best known for its ability to genetically modify host organisms, and it is also known to form biofilms on both abiotic and plant surfaces. Central to the processes of virulence and biofilm formation is the problem of surface attachment. To investigate the process of surface attachment, a screen was performed for mutants deficient in this process. Two separate mutations were isolated in a gene called exoR, found to be homologous to the exoR gene initially studied in the plant symbiont Sinorhizobium meliloti. In the current work, A. tumefaciens ExoR has been found to play a critical role in surface attachment, biofilm formation, exopolysaccharide production, motility, virulence and a number of additional functions. DNA microarray analysis of the ExoR regulon revealed a large network of regulated genes (~600 genes), some activated and others repressed by ExoR. This analysis has provided striking correlation between the mutant phenotypes observed and target genes, indicating negative regulation of exo genes and positive control of motility and chemotaxis-related genes. These experiments also identified a role for ExoR in regulating a Type VI secretion system in A. tumefaciens, which also influences surface attachment. These and other aspects of the exoR phenotype are consistent with its role as a regulatory protein. However, the ExoR protein is not a typical transcriptional regulator, but is predicted to be secreted to the periplasm via an N-terminal secretion signal, and also contains several tetratricopeptide repeat (TPR) domains, thought to mediate protein-protein interactions. Additionally, the role of exopolysaccharide overproduction on the transcriptome was evaluated, resulting in identification of several groups of genes that are indirectly impacted in the exoR mutant through its overproduction of exopolysaccharide.