Inability of Proteus mirabilis mutants to reduce N-oxides and sulfoxide
Valentine, Adeline G.
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Proteus mirabilis strain PM5006 can grow anaerobically on oxidizable substrates such as glycerol or fermentable substrates such as glucose using a number of electron acceptors. It was found that dimethyl sulfoxide (DMSO)/ trimethylamine-N-oxide (TMAO)/ nicotinamide-N-oxide (NAMO), and nitrate (N0₃) can serve as electron acceptors. Cell-free extracts of P. mirabilis were found to reduce these compounds to dimethyl sulfide (DMS), trimethylamine (TMA)/ nicotinamide (NAM), and nitrite (NO₂)/ in the presence of various electron donors such as NADH, formate, lactate, reduced benzyl viologen and sodium dithionite. In order to determine whether the same or different terminal reductase(s) are involved in the reduction of these compounds, the plasmid pJB4JI (cointegrate pPH1::Mu::Tn5), harbored in E. coli 1830, was conjugated into P. mirabilis and transconjugants selected for kanamycin resistance. The Kanamycin-resistant mutants were subsequently screened for their inability to grow anaerobically on glycerol/DMSO medium. When these mutants were tested for anaerobic growth on glycerol medium containing TMAO, NAMO and N0₃ as electron acceptors, it was found that there were two groups. Group I mutants were unable to grow using DMSO, TMAO and NAMO as electron acceptors while their growth was unaffected on NO₃. In contrast, group II mutants were unable to grow on all the electron acceptors including NO₃. Enzyme assays with dithionite-reduced benzyl viologen as electron donor showed that group I mutants were unable to reduce the various electron acceptors except NO₃. The group II mutants lost the ability to reduce all the electron acceptors including NO₃. On the basis of these results we conclude that the same terminal reductase is involved in the reduction of DMSO, TMAO, and NAMO (group I). On the other hand, the group II mutants also lost the NO₃ reductase activity. Thus, group II mutants must be defective in the synthesis or insertion of molybdenum cofactor (MOCO) which is in agreement with previous studies.