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Genetics and Molecular Biology |
The Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Norwich NR4 7UH, UK1
Author for correspondence: J. Maxwell Dow. Tel: +44 1603 452571. Fax: +44 1603 250024. e-mail: max.dow{at}bbsrc.ac.uk
The synthesis of extracellular enzymes and extracellular polysaccharide (EPS) in Xanthomonas campestris pathovar campestris (Xcc) is subject to co-ordinate regulation by a cluster of genes called rpf (for regulation of pathogenicity factors). These genes are located within a 21·9 kb region of the chromosome isolated as the cosmid clone pIJ3020. The genes in the left-hand section of this region of the chromosome have previously been characterized. This paper reports on the genes in the right-hand section and on the phenotypes of mutants with transposon insertions in these genes. Sequence analysis identified eight genes or ORFs with the gene order rpfD–orf1–orf2–orf3–orf4–recJ–rpfE–greA. RecJ and GreA have established functions in recombination and transcriptional elongation, respectively. rpfD encoded a protein with some amino acid sequence relatedness to a hypothetical protein from Caulobacter crescentus and an autolysin response regulator in Bacillus subtilis. The predicted protein products of orf1, 2 and 3 were related to each other and had substantial amino acid sequence relatedness to hypothetical proteins from C. crescentus. Transposon insertions in orf1, 2 and 3 had no effect on the synthesis of extracellular enzymes or EPS. The predicted proteins RpfE and Orf4 showed the highest amino acid sequence relatedness to hypothetical proteins from Bordetella pertussis and Klebsiella pneumoniae, respectively. Transposon insertions in rpfE led to reduced levels of some extracellular enzymes (endoglucanase and protease) and increased levels of others (polygalacturonate lyase). Transposon insertions in orf4 had no effect on polygalacturonate lyase but led to reduced levels of protease and endoglucanase. Levels of EPS were reduced in both rpfE and orf4 mutants. These alterations in the levels of extracellular enzymes, which were relatively modest (between two- and threefold), did not affect the pathogenicity of Xcc on turnip. It is proposed that the gene designation should be rpfI for orf4.
Keywords: Xanthomonas campestris, regulation of pathogenicity factors, rpf gene cluster
Abbreviations: DSF, diffusible signal factor; EPS, extracellular polysaccharide; PGL, polygalacturonate lyase
The EMBL accession number for the sequence reported in this paper is AJ245997.
a Present address: Molecular Genetics Laboratory, Guangxi University, Guangxi 530005, PR China.
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