Regulatory roles of the GacS/GacA two-component system in plant-associatedand other Gram-negative bacteria

The sensor kinase GacS and the response regulator GacA are members of a two -component system that is present in a wide variety of Gram-negative bacter ia and has been studied mainly in enteric bacteria and fluorescent pseudomo nads. The GacS/GacA system controls the production of secondary metabolites and extracellular enzymes involved in pathogenicity to plants and animals, biocontrol of soilborne plant diseases, ecological fitness, or tolerance t o stress. A current model proposes that GacS senses a still-unknown signal and activates, via a phosphorelay mechanism, the GacA transcription regulat or, which in turn triggers the expression of target genes. The GacS protein belongs to the unorthodox sensor kinases, characterized by an autophosphor ylation, a receiver, and an output domain. The periplasmic loop domain of G acS is poorly conserved in diverse bacteria. Thus, a common signal interact ing with this domain would be unexpected. Based on a comparison with the tr anscriptional regulator NarL, a secondary structure can be predicted for th e GacA sensor kinases. Certain genes whose expression is regulated by the G acS/GacA system are regulated in parallel by the small RNA binding protein RsmA (CsrA) at a posttranscriptional level. It is suggested that the GacS/G acA system operates a switch between primary and secondary metabolism, with a major involvement of posttranscriptional control mechanisms.