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Unraveling the structure and function of an uncharacterized nuclease with putative DNA activities in Xanthomonas campestris pathovar campestris.

F. Christopher Peritore-Galve: Plant Pathology and Plant-Microbe Biology Section, Cornell University

<div>Black rot of crucifers caused by the gamma-proteobacterium <em>Xanthomonas campestris </em>pv. <em>campestris </em>(<em>Xcc</em>) is an economically devastating bacterial disease of brassica crops with a worldwide distribution. Infections often begin through contaminated seed, and secondary inoculum enters through the hydathodes or plant wounds, colonizing the vascular system. Despite the importance of seed contamination and bacterial survival, there is limited information on the mechanisms used during periods of low bacterial replication. The OLD nuclease has been hypothesized to play a virulence-lifestyle associated role in <em>Xcc </em>by mediating DNA repair under stressful environmental conditions. Preliminary structural analysis indicates that the ATPase domains of OLD family enzymes share structural homology with Rad50, a protein involved in eukaryotic double strand break repair. Additional structural and functional characterization of the C-terminal catalytic regions from <em>Xcc</em> OLD and <em>Burkholderia pseudomallei</em> OLD reveals surprising homology to DnaG primases and bacterial RNAse M5 maturases and supports a two-metal catalysis mechanism for nuclease cleavage. We have generated knockout and complement <em>old </em>gene mutants in <em>Xcc, </em>and these isolates are being functionally characterized for biofilm formation, <em>in vitro </em>growth, epiphytic survival, and virulence to better understand the influence of OLD nuclease. Studying the molecular machinery underlying survival and colonization may enable the identification of new targets for resistance breeding or chemical control.</div>