Essential role of superoxide dismutase on the pathogenicity of Erwinia chrysanthemi strain 3937

The sodA gene from Erwinia chrysanthemi strain 3937 was cloned by functiona l complementation of an Escherichia coli sodA sodB mutant and sequenced. We identified a 639bp open reading frame, which encodes a protein that is 85% identical to the E, coli manganese-containing superoxide dismutase MnSOD. Promoter elements of this gene were identified by transcriptional mapping e xperiments. We constructed an E. chrysanthemi Delta sodA mutant by reverse genetics. The Delta sodA mutation resulted in the absence of a cytoplasmic SOD, which displays the same characteristics as those of MnSOD. The Delta s odA mutant was more sensitive to paraquat than the wild-type strain. This m utant could macerate potato tubers, similar to the wild-type strain. In con trast, when inoculated on African violets, the mutant produced, at most, on ly small necrotic lesions. If the inoculum was supplemented with the supero xide anion-scavenging metalloporphyrin MnTMPyP or purified SOD and catalase , the Delta sodA mutant was able to macerate the inoculated zone. Generatio n of superoxide anion by African violet leaves inoculated with E. chrysanth emi was demonstrated with nitroblue tetrazolium as an indicator. Therefore, at the onset of infection, E. chrysanthemi cells encounter an oxidative en vironment and require active protective systems against oxidative damages s uch as MnSOD to overcome these types of conditions.