IRON SUPEROXIDE-DISMUTASE OF LEGIONELLA-PNEUMOPHILA IS ESSENTIAL FOR VIABILITY

Legionella pneumophila, the causative agent of Legionnaires' disease, contains two superoxide dismutases (SODs), a cytoplasmic iron enzyme ( FeSOD) and a periplasmic copper-zinc SOD. To study the role of the FeS OD in L. pneumophila, the cloned EeSOD gene (sodB) was inactivated wit h Tn903dIIlacZ, forming a sodB::lacZ gene fusion. By using this fusion , expression of sodB was shown to be unaffected by a variety of condit ions, including several that influence sod expression in Escherichia c oli: aeration, oxidants, the redox cycling compound paraquat, manipula tion of iron levels in the medium, and the stage of growth. A reproduc ible twofold decrease in sodB expression was found during growth on ag ar medium containing charcoal, a potential scavenger of oxyradicals, i n comparison with growth on the same medium without charcoal. No induc tion was seen during growth in human macrophages. Additional copies of sodB(+) in trans increased resistance to paraquat. Construction of a sodB mutant was attempted by allelic exchange of the sodB::lacZ fusion with the chromosomal copy of sodB. The mutant could not be isolated, and the allelic exchange was possible only if wild-type sodB was prese nt in trans. These results indicate that the periplasmic copper-zinc S OD cannot replace the FeSOD. The data strongly suggest that sodB is an essential gene and that FeSOD is required for the viability of L. pne umophila. In contrast, Sod- mutants of E. coli and Streptococcus mutan s grow aerobically and SOD is not required for viability in these spec ies.