New insight into the photoheterotrophic growth of the isocytrate lyase-lacking purple bacterium Rhodospirillum rubrum on acetate

Abstract

Purple non-sulfur bacteria are well known for their metabolic versatility. One of these bacteria, Rhodospirillum rubrum S1H, has been selected by the European Space Agency to ensure the photoheterotrophic assimilation of volatile fatty acids in its regenerative life support system, MELiSSA. Here, we combined proteomic analysis with bacterial growth analysis and enzymatic activity assays in order to better understand acetate photoassimilation. In this isocitrate lyaselacking organism, the assimilation of two-carbon compounds cannot occur through the glyoxylate shunt, and the citramalate cycle has been proposed to fill this role, while, in Rhodobacter sphaeroides, the ethylmalonyl-CoA pathway is used for acetate assimilation. Using proteomic analysis, we were able to identify and quantify more than 1700 unique proteins, representing almost one-half of the theoretical proteome of the strain. Our data reveal that a pyruvate : ferredoxin oxidoreductase (NifJ) could be used for the direct assimilation of acetyl-CoA through pyruvate, potentially representing a new redox-balancing reaction. We additionally propose that the ethylmalonyl-CoA pathway could also be involved in acetate assimilation by the examined strain, since specific enzymes of this pathway were all upregulated and activity of crotonyl-CoA reductase/carboxylase was increased in acetate conditions. Surprisingly, we also observed marked upregulation of glutaryl-CoA dehydrogenase, which could be a component of a new pathway for acetate photoassimilation. Finally, our data suggest that citramalate could be an intermediate of the branched-chain amino acid biosynthesis pathway, which is activated during acetate assimilation, rather than a metabolite of the so-called citramalate cycle.

Bacterial Survival under Extreme UV Radiation: A Comparative Proteomics Study of Rhodobacter sp., Isolated from High Altitude Wetlands in Chile

“…This could be explained by the fact that at high oxygen concentration (∼21%), this photoheterotrophic bacterium would lack photosynthetic complexes and uses aerobic respiration for energy generation (Zeller et al, 2005) due to a high sensitivity of PS to oxidative damage (Nishiyama et al, 2005). Light-regulated metabolic versatility in this photoheterotrophic strain could also be observed in a down-regulation of the enzyme acetyl-coA synthetase (RH2_00279, ratio = 0.66, UVA+UVB condition), involved in acetate assimilation, indicating that during the PAR light condition (used as control), this alternative pathway would have an important role in anabolic processes (Imam et al, 2013; Leroy et al, 2015). …”

Carbon roadmap from syngas to polyhydroxyalkanoates in Rhodospirillum rubrum

“…PFOR catalyses the reverse carboxylation of acetyl‐CoA to generate pyruvate and the rate of the reaction depends exclusively on the concentration of acetyl‐CoA and pyruvate. PFOR activity is expected to be stimulated in the presence of acetate in the cultures (Leroy et al ., ). A control experiment was performed with cells growing in the presence of acetate 30 mM showing that this step is functional in these growth conditions (Fig.…”

Global Proteomic Analysis Reveals High Light Intensity Adaptation Strategies and Polyhydroxyalkanoate Production in Rhodospirillum rubrum Cultivated With Acetate as Carbon Source

“…rubrum. We demonstrated that the duration of the lag phase is linked with inoculum size, suggesting that high light intensity could be deleterious to diluted cells (Leroy et al, 2015). We proposed that high light intensity, resulting from the dilution of the cells at inoculation, could result in an intracellular imbalance in the cofactor pool.…”

“…It is now well accepted that in organisms lacking isocitrate lyase gene (icl − ) (Kornberg and Krebs, 1957), such as Rs. rubrum, an alternative pathway is used for acetate assimilation (i.e., the ethylmalonyl-CoA pathway) (Leroy et al, 2015). In addition to this pathway, the implication of other pathways in acetate photoheterotrophic assimilation has been proposed, and further studies are still needed to attest their possible involvement in acetate assimilation.…”

“…In addition to this pathway, the implication of other pathways in acetate photoheterotrophic assimilation has been proposed, and further studies are still needed to attest their possible involvement in acetate assimilation. Among them, our group reported the functions of pyruvate ferredoxin oxidoreductase (PFOR) and glutaryl-CoA dehydrogenase (Leroy et al, 2015). Recently, our lab investigated the acclimation occurring after long-term cultivation (50 generations) of the wild-type strain of Rs.…”