MntS Helps Facilitate Manganese Delivery To Enzymes In Escherichia coli

Manganese is an essential micronutrient for many microorganisms.  When Escherichia coli is deficient in iron or stressed for hydrogen peroxide, it uses manganese rather than iron to metallate mononuclear enzymes.  It is unclear how these cells traffic manganese to these proteins, since these cells maintain manganese at concentrations below those of most other metals.  Recent identification and characterization of a novel manganese homeostasis gene, mntS, by Waters and Storz sparked our interest in this problem.  They have shown that the mntS  RNA encodes a small protein of 42 amino acids that is repressed by the transcriptional regulator, MntR, in response to high intracellular manganese levels.  We hypothesized that MntS functions as a chaperone that makes manganese more available to proteins.  Here, we tested whether mntS  assists the activation of two manganese-dependent enzymes:  manganese superoxide dismutase (MnSOD) and manganese ribonucleotide reductase (NrdEF).  We show that non-stressed cells lacking mntS  possess 2-fold less active MnSOD compared to wildtype cells, indicating that only a small fraction of protein has manganese bound.  This is not true during hydrogen peroxide stress when intracellular manganese is very abundant; therefore, it seems that mntS  is needed when manganese is scarce.  We also show that the deletion of mntS during iron-limited growth leads to a diminution of NrdEF function, while overexpression of mntS  leads to increased function.  Thus, MntS helps NrdEF gain activity.  Taken together, this study reveals for the first time that cells might use a small protein to facilitate the delivery of manganese to multiple enzymes.