Regulation of autotrophic and heterotrophic metabolism in Pseudomonas oxalaticus OX1: Growth on mixtures of acetate and formate in continuous culture

Growth of Pseudomonas oxalaticus in carbon- and energy-limited continuous cultures with mixtures of acetate and formate resulted in the simultaneous utilization of both substrates at all dilution rates tested. During growth on these mixtures, acetate repressed the synthesis of ribulosebisphosphate carboxylase. The degree of this repression was dependent on the dilution rate and on the ratio of acetate and formate in the medium reservoir. At fixed acetate and formate concentrations in the inflowing medium of 30 and 100 mM, respectively, and dilution rates above 0.10h^-1, the severe repression of autotrophic enzymes resulted in a marked increase in bacterial dry weight compared to the growth yield of the organisms on the two substrates separately. Also, at these dilution rates a significant increase in isocitrate lyase activity was observed in the cells as compared to growth on acetate alone. This indicated that under these conditions more acetate was assimilated and less dissimilated since acetate was partly replaced by formate as the energy source. When formate was added to the reservoir of an acetate-limited culture (S_R=30 mM), derepression of RuBPCase synthesis was observed at formate concentrations of 50 mM and above. Below this concentration formate only served as an energy source for acetate assimilation; when its concentration was increased above 50 mM a progressively increasing contribution of carbon dioxide fixation to the total carbon assimilation was observed as the activity of RuBPCase in the cells increased. It is concluded that in Pseudomonas oxalaticus the synthesis of enzymes involved in autotrophic carbon dioxide fixation via the Calvin cycle is regulated by a repression/derepression mechanism.Abbreviations RuBPCase ribulosebisphosphate carboxylase - PMS phenazine methosulphate - DCPIP 2,6-dichlorophenol-indophenol - FDH formate dehydrogenase - S_R concentration of growth-limiting substrate in reservoir