Nitrogen Starvation Acclimation in Synechococcus elongatus: Redox-Control and the Role of Nitrate Reduction as an  Electron Sink

Klotz, Alexander; Reinhold, Edgar; Doello, Sofia; Forchhammer, Karl

doi:10.3390/life5010888

Cited by 31 publications

(21 citation statements)

References 52 publications

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“…A decrease in amino acid pools with a corresponding increase in AKG is consistent with an NADPH deficit in ΔrpaA cells. Additionally, elevated AKG can activate a nitrogen-starvation transcriptional response in cyanobacteria (36,37), and although these cells were not nitrogen-starved, we found that elevated AKG levels in ΔrpaA were accompanied by this transcriptional response ( Fig. S3 and SI Text).…”

Section: Darkness Initiates Pigmentation Changes and Rapid Cell Deathmentioning

confidence: 68%

Redox crisis underlies conditional light–dark lethality in cyanobacterial mutants that lack the circadian regulator, RpaA

Diamond

Rubin

Shultzaberger

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Cyanobacteria evolved a robust circadian clock, which has a profound influence on fitness and metabolism under daily lightdark (LD) cycles. In the model cyanobacterium Synechococcus elongatus PCC 7942, a functional clock is not required for diurnal growth, but mutants defective for the response regulator that mediates transcriptional rhythms in the wild-type, regulator of phycobilisome association A (RpaA), cannot be cultured under LD conditions. We found that rpaA-null mutants are inviable after several hours in the dark and compared the metabolomes of wild-type and rpaA-null strains to identify the source of lethality. Here, we show that the wild-type metabolome is very stable throughout the night, and this stability is lost in the absence of RpaA. Additionally, an rpaA mutant accumulates excessive reactive oxygen species (ROS) during the day and is unable to clear it during the night. The rpaA-null metabolome indicates that these cells are reductant-starved in the dark, likely because enzymes of the primary nighttime NADPH-producing pathway are direct targets of RpaA. Because NADPH is required for processes that detoxify ROS, conditional LD lethality likely results from inability of the mutant to activate reductant-requiring pathways that detoxify ROS when photosynthesis is not active. We identified second-site mutations and growth conditions that suppress LD lethality in the mutant background that support these conclusions. These results provide a mechanistic explanation as to why rpaA-null mutants die in the dark, further connect the clock to metabolism under diurnal growth, and indicate that RpaA likely has important unidentified functions during the day.C yanobacteria are both key agents of global carbon and nitrogen cycles and promising platforms for renewable chemicals, fuels, and nutraceuticals (1-3). Understanding the control mechanisms that govern the flow of carbon and nitrogen through these organisms is crucial for predicting their behavior in natural environments as well as for improving engineering strategies. Although the basic pathways for carbon and nitrogen metabolism, and their regulation, are well understood in heterotrophic bacteria, cyanobacteria exhibit important deviations in these core metabolic pathways (4-7). Additionally, metabolic control mechanisms in cyanobacteria evolved to be compatible with photoautotrophic metabolism and the dramatic shifts that are imposed on those pathways by predictable daily light-dark (LD) cycles. Examples include enzymatic activity that responds to light-dependent cellular redox changes (8-11); the preference for NADPH, the reductant produced by the photochemical reactions, over NADH by many biosynthetic enzymes (12, 13); and a circadian clock that drives 24-h transcriptional rhythms in most genes (14-16).A daily LD cycle presents a strong metabolic driver for the photosynthetic cyanobacteria, but a circadian clock also imposes daily cycles in transcription and redox regulatory systems (17, 18). Circadian measurements historically have been performed in...

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Section: Darkness Initiates Pigmentation Changes and Rapid Cell Deathmentioning

confidence: 68%

Redox crisis underlies conditional light–dark lethality in cyanobacterial mutants that lack the circadian regulator, RpaA

Diamond

Rubin

Shultzaberger

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…Reduction in the amount of NblB under starvation suggests an active signal for proteolysis of this protein when the level of phycobilisome pigments decreases. In contrast, the nblA gene, encoding a small protein essential for phycobilisome degradation, is highly induced under nutrient starvation (Schwarz and Grossman, ; Sauer et al ., ; Luque et al ., ; van Waasbergen et al ., ; Osanai et al ., ; Sendersky et al ., ; Lahmi et al ., ; Salinas et al ., ; Zabulon et al ., ; Leganes et al ., ; Klotz et al ., ).…”

Section: Resultsmentioning

confidence: 97%

Decomposition of cyanobacterial light harvesting complexes: NblA‐dependent role of the bilin lyase homolog NblB

2018

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Phycobilisomes, the macromolecular light harvesting complexes of cyanobacteria are degraded under nutrient-limiting conditions. This crucial response is required to adjust light excitation to the metabolic status and avoid damage by excess excitation. Phycobilisomes are comprised of phycobiliproteins, apo-proteins that covalently bind bilin chromophores. In the cyanobacterium Synechococcus elongatus, the phycobiliproteins allophycocyanin and phycocyanin comprise the core and the rods of the phycobilisome, respectively. Previously, NblB was identified as an essential component required for phycocyanin degradation under nutrient starvation. This protein is homologous to bilin-lyases, enzymes that catalyze the covalent attachment of bilins to apo-proteins. However, the nblB-inactivated strain is not impaired in phycobiliprotein synthesis, but rather is characterized by aberrant phycocyanin degradation. Here, using a phycocyanin-deficient strain, we demonstrate that NblB is required for degradation of the core pigment, allophycocyanin. Furthermore, we show that the protein NblB is expressed under nutrient sufficient conditions, but during nitrogen starvation its level decreases about two-fold. This finding is in contrast to an additional component essential for degradation, NblA, the expression of which is highly induced under starvation. We further identified NblB residues required for phycocyanin degradation in vivo. Finally, we demonstrate phycocyanin degradation in a cell-free system, thereby providing support for the suggestion that NblB directly mediates pigment degradation by chromophore detachment. The dependence of NblB function on NblA revealed using this system, together with the results indicating presence of NblB under nutrient sufficient conditions, suggests a rapid mechanism for induction of pigment degradation, which requires only the expression of NblA.

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“…To get a clearer picture on the role of Slr1658 in the Synechocystis 6803 regulatory network we selected a number of representative transcripts and stress conditions for further analysis. In addition to pgr5 we tested the iron‐stress inducible transcript isiA (Dühring et al ., ) and the nitrogen limitation inducible nblA transcript (Klotz et al ., ). These three genes exhibited significantly altered expression profiles during Fe limitation (Table ).…”

Section: Resultsmentioning

confidence: 97%

Resequencing of a mutant bearing an iron starvation recovery phenotype defines Slr1658 as a new player in the regulatory network of a model cyanobacterium

et al. 2017

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Photosynthetic microorganisms encounter an erratic nutrient environment characterized by periods of iron limitation and sufficiency. Surviving in such an environment requires mechanisms for handling these transitions. Our study identified a regulatory system involved in the process of recovery from iron limitation in cyanobacteria. We set out to study the role of bacterioferritin co-migratory proteins during transitions in iron bioavailability in the cyanobacterium Synechocystis sp. PCC 6803 using knockout strains coupled with physiological and biochemical measurements. One of the mutants displayed slow recovery from iron limitation. However, we discovered that the cause of the phenotype was not the intended knockout but rather the serendipitous selection of a mutation in an unrelated locus, slr1658. Bioinformatics analysis suggested similarities to two-component systems and a possible regulatory role. Transcriptomic analysis of the recovery from iron limitation showed that the slr1658 mutation had an extensive effect on the expression of genes encoding regulatory proteins, proteins involved in the remodeling and degradation of the photosynthetic apparatus and proteins modulating electron transport. Most significantly, expression of the cyanobacterial homologue of the cyclic electron transport protein PGR5 was upregulated 1000-fold in slr1658 disruption mutants. pgr5 transcripts in the Δslr1658 mutant retained these high levels under a range of stress and recovery conditions. The results suggest that slr1658 is part of a regulatory operon that, among other aspects, affects the regulation of alternative electron flow. Disruption of its function has deleterious results under oxidative stress promoting conditions.

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Nitrogen Starvation Acclimation in Synechococcus elongatus: Redox-Control and the Role of Nitrate Reduction as an Electron Sink

Cited by 31 publications

References 52 publications

Redox crisis underlies conditional light–dark lethality in cyanobacterial mutants that lack the circadian regulator, RpaA

Redox crisis underlies conditional light–dark lethality in cyanobacterial mutants that lack the circadian regulator, RpaA

Decomposition of cyanobacterial light harvesting complexes: NblA‐dependent role of the bilin lyase homolog NblB

Resequencing of a mutant bearing an iron starvation recovery phenotype defines Slr1658 as a new player in the regulatory network of a model cyanobacterium

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