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hok-sok toxin-antitoxin system plays important roles in morphological plasticity, bacterial persistence, and catalase activity in Erwinia amylovora

Jingyu Peng: Michigan State University

<div><em>Erwinia amylovora </em>is a plant pathogenic bacterium that causes devastating fire blight disease on apple, pear and cherry. Recently, we identified a chromosomally-encoded <em>hok</em>-<em>sok </em>toxin-antitoxin system in <em>E. amylovora </em>Ea1189. The ectopic expression of <em>hok </em>in <em>E. amylovora</em> is highly toxic and turns a subpopulation of cells into the viable but non-culturable (VBNC) state. Overexpression of small RNA <em>sok</em> partially reversed the toxicity of <em>hok</em> by indirectly inhibiting the translation of <em>mok</em>, a modulator gene of <em>hok</em>. A <em>hok-sok</em> knockout mutant,<em> Δhok-sok</em>, exhibited slower exponential growth than the WT. Using scanning electron microscopy, we observed that subpopulations of the <em>Δhok-sok </em>mutant had switched to filamentous growth with the longest cell observed measuring 42 nm in length compared to 2 nm in the WT. The <em>hok</em>-<em>sok</em> locus appears to be important for bacterial persistence. In both the WT and an <em>obg</em> (a GTPase gene that is important for bacterial persistence in <em>E. coli</em>) overexpression background, the <em>Δhok-sok </em>strain exhibited a greatly reduced number of persister cells under several antibiotics tested. <em>Δhok-sok </em>also had significantly-impaired catalase activity, and we are currently investigating the <em>hok-sok</em> locus-linked crosstalk between catalase activity and persister cell formation. Taken together, <em>E. amylovora</em> retains the chromosomal <em>hok-sok </em>locus, a potentially self-toxic system, in exchange for advantages that could affect environmental fitness.</div>