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Characterization of a catalase gene from Aeromonas veronii, the digestive-tract symbiont of the medicinal leech

¹ Department of Molecular and Cell Biology, University of Connecticut, 91 N. Eagleville Road, Unit-3125, Storrs, CT 06269-3125, USA
² Institute for Infectious Diseases, University of Berne, Friedbühlstr. 51, CH-3010 Berne, Switzerland

Correspondence
Joerg Graf
joerg.graf{at}uconn.edu

The catalase gene katA of the medicinal leech symbiont Aeromonas veronii bv. sobria was cloned, sequenced, and functionally characterized. Southern hybridization, using an A. veronii katA-specific hybridization probe, suggested the presence of a single gene copy in many Aeromonas species. A. veronii katA consisted of 1446 nt encoding a protein with a high degree of similarity to the small-subunit group III bacterial catalases. A catalase-null mutant (JG186) was constructed through gene-replacement mutagenesis. In the parent strain (HM21R), catalase activity was only detected in extracts of cells grown to early exponential phase following H₂O₂ induction, in which the ability to induce activity was inversely related to optical density. In contrast, induced JG186 cells were very sensitive to oxidative stress, with survival being affected even at low H₂O₂ concentrations. In contrast to the findings of previous reports of other symbiotic systems, the catalase mutant was not defective in its ability to competitively colonize or persist within its host, in both co-inoculation and sole-colonization assays. This body of evidence suggests either that oxidative stress, in the form of H₂O₂ exposure, is not encountered by the microbial partner under the examined symbiotic conditions or that compensatory mechanisms exist. The data suggest that although many colonization factors reoccur, each symbiotic system has also evolved specific mechanisms that affect symbiont–host dynamics.

The GenBank/EMBL/DDBJ accession no. for the A. veronii katA sequence is EF028076.

A figure showing the results of transformation of E. coli ZK918 with purified plasmid DNA from the A. veronii genomic library, after plating on MacConkey agar, is available as supplementary data with the online version of this paper.

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