Graduate Thesis Or Dissertation
 

Characterization of the pathogenicity and phylogeny of members of the Rhodococcus genus

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  • Plant pathogenic Rhodococcus species are persistent pathogens able to cause severe growth deformities on a large range of hosts. The most well studied species, R. fascians is predicted by current models to synthesize a mixture of cytokinins that act to directly perturb the hosts' hormone balances, which results in abnormal growth. The biosynthesis and modification of cytokinins has been attributed to proteins encoded by the fas locus. The fasD gene encodes an isopentenyl transferase, a key enzyme in cytokinin biosynthesis, and is necessary for pathogenicity. In addition to fasD, fasA, putatively encoding a P450 monooxygenase, and fasF, encoding a phosphoribohydrolase, are hypothesized to be necessary for pathogenicity. The work presented in this thesis employs the use of whole genome sequences from over 20 isolates of Rhodococcus to challenge the current virulence model and develop new models. Counter to the current virulence model, our data shows R. fascians produces only one active cytokinin in a fas-dependent manner. Analysis of the genome sequences revealed a unique pathogenic isolate that lacks the fas operon. However, this isolate contains a gene predicted to encode a protein with isopentenyl transferase and phosphoribohydrolase domains, putatively capable of catalyzing and activating cytokinins, respectively. We also developed methods to generate non-polar gene deletion mutants of fasA and fasF, two key genes in the extant virulence model. Characterization of these mutants indicates fasA and fasF are not necessary for pathogenicity. Finally, we used the genome sequences from 59 Rhodococcus isolates, including the 21 genomes we sequenced, to provide a framework for resolving the Rhodococcus genus.
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