| Summary: | 碩士 === 慈濟大學 === 微生物學免疫學暨生物化學碩士班 === 100 === phiLf is a filamentous phage that specifically infects Xanthomonas campestris pv. campestris (Xcc), a gram-negative plant pathogenic bacterium causing black rot disease in crucifers. Xcc produces large amounts of an exopolysaccharide (called xanthan gum) and an array of extracellular enzymes including amylase, pectinases, cellulases and proteases. These extracellular substances have been implicated as virulence factors involved in pathogenesis. Secretion of extracellular enzymes in this bacterium requires the type II secretion machinery, with the xps cluster being the major genes participating in the secretion. It is known that XpsD, encoded by xpsD, is an outer membrane protein that forms a channel (secretin) containing at least 12 mature XpsD polypeptides to facilitate the secretion of extracellular enzymes across the outer membrane. Interestingly, XpsD shares homology with pIV, the outer membrane protein encoded by filamentous phage Ff of Escherichia coli (f1, fd and M13), required for exporting Ff phage particles. The strongest homology is found at their COOH termini of approximately 200 amino acids residues. Although phiLf has an almost identical genome organization as Ff, this Xcc phage lacks the gene analogous to the Ff gIV. These findings suggest that XpsD may play a role analogous to that of pIV in phage export. Based on this prediction, the aims of this study are to test whether 1) XpsD is required for phage export by testing with xpsD mutant, 2) the wild-type phenotype can be restored by complementation with cloned wild-type xpsD gene, and 3) cloned gIV can substitute for the function of XpsD in phage export, should XpsD be shown to have the desired function.
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