Browsing by Author "Ding, Hao"

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    Plasmid biology of rhizobium leguminosarum: novel conjugation system and glycerol catabolic genes
    (2012) Ding, Hao; Hynes, Michael F.
    Large plasmids in rhizobia play important roles in rhizobia-legume symbioses and bacterial survival and competitiveness in the rhizosphere. In R. leguminosarum bv. viciae VF39SM, a novel conjugation system on plasmid pRleVF39b was isolated and found to be different from the rhizobial quorum sensing-regulated or the RctA-repressed conjugation systems. The entire transfer region on pRleVF39b encompasses a trb-like operon and traG, encoding the mating pore formation component and the coupling protein, and a relaxase gene traA located 9 kb downstream of the trb operon. Mutations in any of the above genes completely abolished the transfer of pRleVF39b. The transfer of pRleVF39b was affected mainly by its host genetic background. Under the conditions tested, pRle VF3 9b transferred at highest frequency at about 10-4 transconjugants per recipient from plasmid-free Agrobacterium tumefaciens UBAPF2 to another Agrobacterium recipient. The transfer frequency decreased about 10 to 100-fold when Rhizobium was the donor, and decreased at least 1000-fold when Rhizobium was the recipient. An xre-type transcriptional repressor gene trbR was identified in the region between traG and traA. Mutation in trbR led to SO-fold increases in trb operon expression and pRleVF39b transfer. A glycerol uptake and catabolic locus on pRleVF39c was found to comprise a glpR gene, encoding a transcriptional regulator, and the glp operon, including a glpD gene, encoding glycerol 3-phosphate dehydrogenase, glpSTPQUV encoding an ATP binding cassette transporter, and a glpK gene encoding glycerol kinase. All the genes within the operon were required for growth on glycerol, except for glpK whose function can be partially complemented by a glpKch gene on the chromosome. The glp operon was inducible by glycerol, glycerol 3-phosphate, and pea seed exudates. GlpR represses the expression of the glp operon in the absence of inducer. Glycerol uptake in Rhizobium was shown to be an active process mediated by an ABC transporter. Mutants that are unable to utilize glycerol as a sole carbon source were less competitive in nodulating peas than the wild-type, indicating that glycerol catabolism might confer advantages to the bacterium in the rhizosphere or in plants.