ENZYMES OF THE REDUCTIVE CITRIC-ACID CYCLE IN THE AUTOTROPHIC EUBACTERIUM AQUIFEX-PYROPHILUS AND IN THE ARCHAEBACTERIUM THERMOPROTEUS-NEUTROPHILUS

The autotrophic carbon fixation pathway was studied in the thermophili c hydrogen oxidizing eubacterium Aquifex pyrophilus and in the thermop hilic sulfur reducing archaebacterium Thermoproteus neutrophilus. Neit her organism contained ribulose-1,5-bisphosphate carboxylase activity suggesting that the Calvin cycle is not operating. Rather, all enzymes of the reductive citric acid cycle were found in A. pyrophilus. In T. neutrophilus ATP citrate lyase activity was detected which has not be en achieved so far; this finding corroborates earlier work suggesting the presence of the reductive citric acid cycle in this archaebacteriu m. The reductive citric acid cycle for autotrophic CO2 fixation now ha s been documented in the eubacterial branches of the proteobacteria, i n green sulfur bacteria, and in the thermophilic Knallgas bacteria as well as in the branch of the sulfur dependent archaebacteria.