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Summary

A genomic island (GEI) of the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough, found to be able to migrate between two tRNA-Met loci of the genome, contains genes for rubredoxin:oxygen oxidoreductase-1 (roo1) and hybrid cluster protein-1 (hcp1) with additional copies for these genes (roo2 and hcp2) being found elsewhere on the chromosome. A suite of mutants was created in which roo2 and/or hcp2 and/or the GEI were either present or missing. The GEI and roo2 increased survival under microaerobic conditions and allowed growth in closer proximity to the air–water interface of soft agar tubes, two properties which appeared to be closely linked. When Hcp2+ GEI+ or Hcp2 GEI+ cells, harbouring cytochrome c nitrite reductase (NrfHA) and growing on lactate and sulfate, were amended with 10 mM nitrite at mid-log phase (8–10 mM sulfide), all nitrite was reduced within 30 h with a rate of 3.0 mmol (g biomass)−1 h−1 after which sulfate reduction resumed. However, Hcp2+ GEI or Hcp2 GEI cells were unable to use lactate, causing sulfide to be used as electron donor for nitrite reduction at a sixfold lower rate. Complementation studies indicated that hcp1, not roo1, enhanced the rate of nitrite reduction under these conditions. Hcp2 enhanced the rate of nitrite reduction when, in addition to lactate, hydrogen was also present as an electron donor. These results indicate a critical role of Hcps in alleviating nitrite stress in D. vulgaris Hildenborough by maintaining the integrity of electron transport chains from lactate or H2 to NrfHA through removal of reactive nitrogen species. It thus appears that the GEI contributes considerably to the fitness of the organism, allowing improved growth in microaerobic environments found in sulfide–oxygen gradients and in environments, containing both sulfide and nitrite, through the action of Roo1 and Hcp1 respectively.