A novel plant pathogenesis regulatory system in Erwinia: functional analysis of a new post-transcriptional input to bacterial quorum sensing control.

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This proposed research involves the study of a new gene in a bacterial pathogen of plants. The bacterial pathogen (Erwinia) causes rotting diseases of potato and other plants and is important in potato crop production. The bacteria attack the plant by producing a spectrum of enzymes that can degrade the cell walls of the plant leading to the commercially-significant rotting disease symptoms. The ability of the bacteria to make these plant cell wall degrading enzymes is affected by the cell population density of the bacteria; the enzymes are only made in abundance when the bacterial density if high. The bacteria make a small diffusible chemical signal called OHHL and the concentration of this molecule is a direct reflection of bacterial density. In this process (quorum sensing) the bacteria use the chemical signal to communicate with each other and thereby link cell density to the ability to aggressively rot the plant. If the ability to enact the process of quorum sensing is blocked then the bacteria are no longer capable of causing disease. Consequently, a deeper understanding of the mode of action of quorum sensing in this potato pathogen is important for our fundamental appreciation of how bacterial pathogens communicate with each other and, if we can understand the process, how we might intervene in the disease. There are no chemical control systems available for potato diseases caused by Erwinia and so fundamental understanding of how it causes infection is the only route to an eventual rational therapy. In this proposal we will investigate the role of a new gene (ECA2020) that we have shown recently to be involved in the process of quorum sensing. The product of this gene affects the expression/functionality of one of the key proteins (VirR) involved in quorum sensing. We will study the way ECA2020 operates by investigating which other proteins it interacts with and we will test the hypothesis that it may have some functions similar to proteins from higher organisms that are involved in messenger RNA processing. We will look for bacterial mutants that can bypass their dependence on the ECA2020 system and we will study the precise impact on production of the fully active VirR protein. We will also test the possibility that the contiguous gene (ECA2019) might be involved in a related process and we will investigate how the ECA2020 gene is itself regulated. The overall aim is to try to work out how this new gene modulates the function of the quorum sensing system during plant infection and disease initiation because this might be a target in the longer term for control of potato rotting diseases caused by Erwinia.