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In phytopathogenic bacteria, the T3SS are encoded by the hrp (hypersensitive response and pathogenicity) genes, which can be divided into two main groups (Tang et al. 2006). The hrp genes of Pseudomonas syringae, Erwinia spp. and Pantoea stewartii belong to group I that is regulated by the HrpL, an ECF (extracytoplasmic factor) family alternate sigma factor (Xiao et al. 1994b). The hrp genes in group II are activated by an AraC-like activator, such as HrpX in Xanthomonas spp. and HrpB in Ralstonia solanacearum (Alfano and Collmer 1997). To date, a large number of T3SS-regulating components acting upstream of hrp genes have been identified and characterized in various phytopathogenic bacteria, including two-component systems (TCS), transcription factors, membrane proteins, quorum-sensing genes, plant-derived compounds and medium components. In order to better understand the molecular basis of bacterial pathogenesis and the microbe-plant interaction, future studies are required to elucidate the nature of T3SS signals, how bacteria sense the signals and the connections between multiple T3SS regulatory genes and hrp/effector genes.

Bacterial type three secretion systems.

Bacteria use at least eight different secretion systems (from type one to type eight) to translocate proteins from the interior cytoplasm cross the membranes to its exterior, which is a very important mechanism for bacteria to adapt surrounding environment and survive (Desvaux et al. 2009). Around 20 Gram-negative pathogenic bacteria rely on the needle-like type three secretion system (T3SS) to secret proteins that help bacteria to infect eukaryotic host organisms (Jin et al. 2003). The T3SS is a sophisticated molecular machinery that is composed of more than 20 different proteins, making it the most complex secretion system. T3SSs are essential for the bacterial virulence, which has been proved by the evidence that T3SS-defecient mutants are nonpathogenic (Zhou and Chai, 2008).英语论文网，代写英语论文，留学生作业

Even though a complete T3SS supramolecular structure has yet to be purified from phytopathogenic bacteria, the elegant work was carried out in the mammalian pathogen Salmonella enterica (Buttner and He, 2009). The T3SS needle structure measures approximately 80 nm in length and 8 nm in width. It starts at the basal body in the bacterial cytoplasm, crosses the two bacterial membranes and extends the needle out of the cell. The basal body is composed of two rings that interact with the inner cytoplasmic and the outer membrane, respectively. An inner rod connects the basal body to the needle, which is made of 100-150 subunits of one single small protein (Kubori et al. 1998). In phytopathogenic bacteria, the T3SS filaments are called hrp (hypersensitive response and pathogenicity) pili (Roine et al. 1997).

The genes encoding the T3SS secretion apparatus are located on the chromosome in some bacteria and on a plasmid in other bacteria. In Pseudomonas syringae, for example, most T3SS genes (called T3 genes hereafter) are located in six operons in a chromosomal hrp island that is responsible for pathogenicity of host plants and the hypersensitive response (HR) of resistant and non-host plants (Collmer et al. 2000). The rest of the T3 genes are scattered in other regions of the genome. On the other hand, Shigella spp. https://www.51lunwen.org/dxygassignment/ has a large virulence plasmid that encodes all T3SS genes (Buchrieser et al. 2000). The T3SS proteins includ本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。