Reductive TCA cycle enzymes and reductive amino acid synthesis pathways contribute to electron balance in aRhodospirillum rubrumCalvin cycle mutant

Abstract

13Purple nonsulfur bacteria (PNSB) use light for energy and organic substrates for carbon and 14 electrons when growing photoheterotrophically. This lifestyle generates more reduced electron 15 carriers than are required for biosynthesis, even during consumption of some of the most 16 oxidized organic substrates like malate and fumarate. Excess reduced electron carriers must be 17 oxidized for photoheterotrophic growth to occur. Diverse PNSB commonly rely on the CO2-18fixing Calvin cycle to oxidize excess reduced electron carriers. Some PNSB also produce H2 or 19 reduce terminal electron acceptors as alternatives to the Calvin cycle. Rhodospirillum rubrum 20Calvin cycle mutants defy this trend by growing phototrophically on malate or fumarate without 21 H2 production or access to terminal electron acceptors. We used 13 C-tracer experiments to 22 examine how a Rs. rubrum Calvin cycle mutant maintains electron balance under such 23 conditions. We detected the reversal of some TCA cycle enzymes, which carried reductive flux 24 from malate or fumarate to α-ketoglutarate. This pathway and the reductive synthesis of amino 25 acids derived from α-ketoglutarate are likely important for electron balance, as supplementing 26 the growth medium with α-ketoglutarate-derived amino acids prevented Rs. rubrum Calvin cycle 27 mutant growth unless a terminal electron acceptor was provided. Flux estimates also suggested 28 that the Calvin cycle mutant preferentially synthesized isoleucine using the reductive threonine-29 dependent pathway instead of the less-reductive citramalate-dependent pathway. Collectively, 30 our results suggest that alternative biosynthetic pathways can contribute to electron balance 31 within the constraints of a relatively constant biomass composition. 32 33 34 35

Carbon substrate re‐orders relative growth of a bacterium using Mo‐, V‐, or Fe‐nitrogenase for nitrogen fixation

“…Proteins related to branched‐chain amino acid metabolism, whose synthesis can re‐oxidize cellular electron carriers for R . palustris during photoheterotrophic growth on succinate but not on acetate (McCully et al ., ), are more abundant in this treatment (Fig. S8).…”