Role and regulation of the Shigella flexneri sit and MntH systems

Shigella flexneri possesses at least two putative high-affinity manganese acquisition systems, SitABCD and MntH. Mutations in the genes encoding the components of both of these systems were constructed in S. flexneri. The sitA mntH mutant showed reduced growth, relative to the wild type, in Luria broth (L broth) containing the divalent metal chelator ethylene diamino-o-dihydroxyphenyl acetic acid, and the addition of either iron or manganese restored growth to the level of the wild-type strain. Although the sitA mntH mutant was not defective in surviving exposure to superoxide generators, it was defective in surviving exposure to hydrogen peroxide. The sitA mntH mutant formed wild-type plaques on Henle cell monolayers but had a reduced ability to survive in activated macrophage lines. Expression of the S. flexneri sit and mntH promoters was higher when Shigella was in Henle cells than when it was in L broth. Expression of both the sit and mntH promoters was repressed by either iron or manganese, and this repression was partially dependent upon Fur and MntR, respectively. The mntH promoter, but not the sit promoter, exhibited OxyR-dependent induction in the presence of hydrogen peroxide.