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Clostridium aceticum was the first isolated autotrophic acetogen, converting CO₂ plus H₂ or syngas to acetate. Its genome has now been completely sequenced and consists of a 4.2-Mbp chromosome and a small circular plasmid of 5.7 kbp. Sequence analysis revealed major differences from other autotrophic acetogens. C.áaceticum contains an Rnf complex for energy conservation (via pumping protons or sodium ions). Such systems have also been found in C.áljungdahlii and Acetobacterium woodii. However, C.áaceticum also contains a cytochrome, as does Moorella thermoacetica, which has been proposed to be involved in the generation of a proton gradient. Thus, C.áaceticum seems to represent a link between Rnf- and cytochrome-containing autotrophic acetogens. In C.áaceticum, however, the cytochrome is probably not involved in an electron transport chain that leads to proton translocation, as no …