Introduction

Ralstonia solanacearum

Ralstonia solanacearum belongs to the family of Gram-negative phyto-pathogens. This bacteria causes great losses in crops worldwide in tropical, subtropical and temperate environments. The genes involved in virulence are known as hrp and they are induced by contact with various plant species from three different families of dicotyledonous (Solanaceae, Cruciferae and legumes), including both host and non host species. These genes encode a type III secretion system (TTSS), required to develop the disease in the host or the hypersensitive response (HR) in resistant and non-host plants. The HR is a defense mechanism that certain plant species possess, in which cells infected by a pathogen are killed to prevent spread to healthy tissue. The secretion system is expressed only when there is a physical interaction between the bacterium and the plant cell. PrhA, the protein that recognizes the plant cell ligand, is an outer membrane receptor that shows homology with some TonB-dependent siderophore receptors. However, PrhA is not involved in the bacteria-plant cell adhesion, but only detects it. The interaction of bacteria with plant cells occurs in two steps: first, Ralstonia binds to the cell surface. This union is independent of PrhA or any protein encoded by hrp. Once the union has been established, the PrhA receiver can detect an accessible ligand, which increases the transcription of the hrp regulatory gene. Induction of hrp expression is very fast, around 90 minutes, a period much shorter than generation time of R. solanacearum in optimal conditions. PrhA fundamental feature is that it is the first known bacterial receptor that can detect a non-diffusible signal present in plant cell walls. The possibility of attaching our bacteria to a specific tissue was what made us choose the Ralstonia system as a candidate for our project. The induction of the expression of virulence hrp regulon integrates a complex signaling cascade that begins in the PrhA outer membrane protein. PrhA transduces the contact-dependent signal through a complex regulatory cascade composed of PrhR, PrhI, PrhJ, HrpG and HrpB. Finally, HrpB activates the expression of hrp, comprising the TTSS structural genes and genes that encode effector proteins that travel through the TTSS.

Systems

Prh system of Ralstonia solanacearum

The Prh system integrates the genes involved in the control of expression of hrp virulence genes from Ralstonia solanacearum. hrp gene encodes a type III secretion system, necessary to develop disease in their hosts. The induction of these genes integrates a complex signaling network that begins when the bacteria and the plant cell contact. This signaling mechanism is composed of PrhR PrhI PrhA proteins and other regulators that, as a last resort, activate the expression of the hrp and hrc genes.

The induction of virulence genes occurs when PrhA contacts with the plant cell. PrhA is an outer membrane protein that recognizes a non-diffusible signal from the plant cell wall. PrhA-ligand binding causes that the periplasmic exposed N-terminal end of FecA interacts with the carboxy terminal end of PrhR (an inner membrane protein) in the periplasm, transmitting the signal across the cytoplasmic membrane. In the cytoplasm inactive PrhI is actived by PrhR interaction.

The prhIR gene expression is induced in coculture with plant cells for unknown environmental signal PrhA independent. PrhI is an ECF sigma factor that, when it is activated, binds to RNA polymerase core enzyme and directs the polimerase to the promoter region of PrhJ to initiate transcription. PrhJ protein induces hrpG transcription, which activates expression of hrpB gene and finally expresses hrp and hrc virulence genes.

The PrhA-PrhR-PrhI module of Ralstonia works similarly to Feca-FecR-FecI module of E.coli, with both similar sequences. PrhA shows homology with several members of the family of siderophore outer membrane receptors (as is the case of FecA). The three boxes that this family of proteins presents (TonB-box, box II and boxIII) are well conserved and correctly located in PrhA. The periplasmic N-terminal extension of FecA that is required for transduction of signals FecR, also appears in PrhA. PrhR has a transmembrane domain (TM) in the same position as FecR and both proteins have a similar orientation. In addition, two of the three tryptophan residues of the N-terminal end of FecR required to activate FecII are present in PrhR. However, unlike most of the siderophores, both prhIR and prhA lack Fur-boxes which are necessary for the regulation in function of the internal iron status.

Another striking difference between PrhAIR and FecAIR is their gene organization: while there is a physical grouping between genes of FecAIR, in PrhAIR system, prhA is at the left edge of hrp gene cluster and prhIR is on the right side of cluster, both prhA and prhIR separated by virulence genes. Moreover, in contrast to the Fec system where FecA is activated by FecI and repressed by Fur, prhA is always expressed a very low level in the presence of the inducing signal and is PrhI independent.

To ensure that our system is able to detect the walls of plant cells and causes the excretion of chemoattractant, in order to be able to achieve effective bacterial accumulation around plant cells, we have designed four different circuits. These attempts to prevent the different problems that arise to us. The first circuit integrates regulatory components PrhA-PrhI-PrhR and PprhJ of R. solanacearum, but they are transfered to E. coli. The genes required for synthesis and excretion of the chemoattractant are under promoter PrhJ.

Because the Prh system is not fully characterized, unknown elements could be involved in and prevent the signal being correctly transmitted to PprhJ. For that reason, we have designed other circuits avoid this signaling cascade.