title: Metabolic Flux Analysis of the Synechocystis sp. PCC 6803 ΔnrtABCD Mutant Reveals a Mechanism for Metabolic Adaptation to Nitrogen-Limited Conditions.
abstract: Metabolic flux redirection during nitrogen-limited growth was investigated in the Synechocystis sp. PCC 6803 glucose-tolerant (GT) strain under photoautotrophic conditions by isotopically non-stationary metabolic flux analysis (INST-MFA). A ΔnrtABCD mutant of Synechocystis sp. PCC 6803 was constructed to reproduce phenotypes arising during nitrogen starvation. The ΔnrtABCD mutant and the wild-type GT strain were cultured under photoautotrophic conditions by a photobioreactor. Intracellular metabolites were labeled over a time course using NaH13CO3 as a carbon source. Based on these data, the metabolic flux distributions in the wild-type and ΔnrtABCD cells were estimated by INST-MFA. The wild-type GT and ΔnrtABCD strains displayed similar distribution patterns, although the absolute levels of metabolic flux were lower in ΔnrtABCD. Furthermore, the relative flux levels for glycogen metabolism, anaplerotic reactions and the oxidative pentose phosphate pathway were increased in ΔnrtABCD. This was probably due to the increased expression of enzyme genes that respond to nitrogen depletion. Additionally, we found that the ratio of ATP/NADPH demand increased slightly in the ΔnrtABCD mutant. These results indicated that futile ATP consumption increases under nitrogen-limited conditions because the Calvin-Benson cycle and the oxidative pentose phosphate pathway form a metabolic futile cycle that consumes ATP without CO2 fixation and NADPH regeneration.
authors.affiliation: Department of Biological Science and Technology, Faculty of Industrial Science and Technology, Tokyo University of Science, Niijuku, Katsushika-ku, Tokyo, Japan.
authors.forename: Tsubasa
authors.initials: T
authors.lastname: Nakajima
authors.affiliation: Faculty of Life and Medical Sciences, Doshisha University, Kyotanabe, Japan.
authors.forename: Katsunori
authors.initials: K
authors.lastname: Yoshikawa
authors.affiliation: Department of Bioinformatic Engineering, Graduate School of Information Science & Technology, Osaka University,  Yamadaoka, Suita, Osaka, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Toya
authors.affiliation: Department of Bioinformatic Engineering, Graduate School of Information Science & Technology, Osaka University,  Yamadaoka, Suita, Osaka, Japan.
authors.forename: Fumio
authors.initials: F
authors.lastname: Matsuda
authors.affiliation: Department of Medical Biology, Akita University Graduate School of Medicine, Akita, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Shimizu
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D064688:Metabolic Flux Analysis / Q000379:methods*; D009154:Mutation / Q000235:genetics; D009584:Nitrogen / Q000172:deficiency / Q000378:metabolism*; D046939:Synechocystis / Q000235:genetics* / Q000378:metabolism*
chemical_list: D000255:Adenosine Triphosphate; D009584:Nitrogen
keywords: ATP/NADPH ratio; Isotopically non-stationary metabolic flux analysis; Metabolic adaptation; Nitrogen starvation; Synechocystis sp. PCC 6803