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The dissimilatory reduction of sulfate is an ancient metabolic process central to today’s biogeochemical cycling of sulfur and carbon in marine sediments. Until now its polyphyletic distribution was most parsimoniously explained by multiple horizontal transfers of single genes rather than by a not-yet-identified “metabolic island.” Here we provide evidence that the horizontal transfer of a gene cluster may indeed be responsible for the patchy distribution of sulfate-reducing prokaryotes (SRP) in the phylogenetic tree. We isolated three DNA fragments (32 to 41 kb) from uncultured, closely related SRP from DNA directly extracted from two distinct marine sediments. Fosmid ws39f7, and partially also fosmids ws7f8 and hr42c9, harbored a core set of essential genes for the dissimilatory reduction of sulfate, including enzymes for the reduction of sulfur intermediates and synthesis of the prosthetic group of the dissimilatory sulfite reductase. Genome comparisons suggest that encoded membrane proteins universally present among SRP are critical for electron transfer to cytoplasmic enzymes. In addition, novel, conserved hypothetical proteins that are likely involved in dissimilatory sulfate reduction were identified. Based on comparative genomics and previously published experimental evidence, a more comprehensive model of dissimilatory sulfate reduction is presented. The observed clustering of genes involved in dissimilatory sulfate reduction has not been previously found. These findings strongly support the hypothesis that genes responsible for dissimilatory sulfate reduction were concomitantly transferred in a single event among prokaryotes. The acquisition of an optimized gene set would enormously facilitate a successful implementation of a novel pathway. Dissimilatory sulfate reduction or sulfate respiration is a key process in the mineralization of organic matter in marine sediments. Up to 50% of organic carbon in coastal sediments is mineralized anaerobically by sulfate-reducing prokaryotes (SRP) (20). This process is one of the oldest types of biological energy conservation. Evidence from geological sulfur isotope records suggests that it arose for the first time approximately 3.5 billion years ago (47). The early origin and appearance of dissimilatory sulfate reduction (DSR) should be reflected in a widespread distribution among prokaryotes and a paralleled phylogeny of the 16S rRNA gene and functional genes. However, this metabolic pathway is patchily scattered and occurs solely within four bacterial and two archaeal lineages (43, 53, 57). Comparative phylogenetic studies on the 16S rRNA gene and the two key enzymes, dissimilatory sulfite reductase (DsrAB) and adenosine-5-phosphosulfate reductase (AprAB), suggested multiple, independent events of horizontal gene transfer (HGT) of the respective functional genes (22). For instance, the DsrAB sequence of Archaeoglobus spp. is more closely

Frequent spontaneous loss of the magnetic phenotype was observed in stationary-phase cultures of the magnetotactic bacteriumMagnetospirillum gryphiswaldenseMSR-1. A nonmagnetic mutant, designated strain MSR-1B, was isolated and characterized. The mutant lacked any structures resembling magnetosome crystals as well as internal membrane vesicles. The growth of strain MSR-1B was impaired under all growth conditions tested, and the uptake and accumulation of iron were drastically reduced under iron-replete conditions. A large chromosomal deletion of approximately 80 kb was identified in strain MSR-1B, which comprised both the entiremamABandmamDCclusters as well as further putative operons encoding a number of magnetosome-associated proteins. A bacterial artificial chromosome clone partially covering the deleted region was isolated from the genomic library of wild-typeM. gryphiswaldense. Sequence analysis of this fragment revealed that all previously identifiedmamgenes were closely linked with genes encoding other magnetosome-associated proteins within less than 35 kb. In addition, this region was remarkably rich in insertion elements and harbored a considerable number of unknown gene families which appeared to be specific for magnetotactic bacteria. Overall, these findings suggest the existence of a putative large magnetosome island inM. gryphiswaldenseand other magnetotactic bacteria.