📄 systems biology

Identifying Novel Targets of the Stringent Response in Plants and Cyanobacteria using chemoproteomics

This study employs chemoproteomics to identify novel ppGpp-binding targets in plants and cyanobacteria, revealing distinct regulatory mechanisms such as the inhibition of pyrimidine metabolism in chloroplasts and the activation of glycogen synthesis and carboxysome aggregation in cyanobacteria, thereby expanding our understanding of how photosynthetic organisms adapt to environmental stress.

Original authors: Karlsson, A., Rillema, R., Sporre, E., Englund, E., Vogiatzi, N., Llavina Ramirez, J., Gurdap, C. O., Sezgin, E., Edfors, F., Blikstad, C., Strand, A., Ducat, D. C., Hudson, E. P.

Published 2026-07-09

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Karlsson, A., Rillema, R., Sporre, E., Englund, E., Vogiatzi, N., Llavina Ramirez, J., Gurdap, C. O., Sezgin, E., Edfors, F., Blikstad, C., Strand, A., Ducat, D. C., Hudson, E. P.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine a photosynthetic organism—like a plant or a cyanobacterium (a tiny, sun-eating bacterium)—as a bustling city powered by sunlight. When the sun goes down or food runs low, this city needs a "Code Red" alarm system to switch from "growth mode" to "survival mode." That alarm is a tiny chemical messenger called ppGpp.

For decades, scientists knew this alarm existed in bacteria like E. coli, but they were flying blind when it came to plants and cyanobacteria. They knew the alarm rang, but they didn't know exactly which doors it was unlocking or which locks it was jamming. This paper acts like a high-tech detective squad, using a method called chemoproteomics (think of it as a "thermal fingerprinting" technique) to find out exactly which proteins in the city are shaking hands with the ppGpp alarm.

Here is what they found, broken down into the city's most important neighborhoods:

1. The Pyrimidine Factory (Plants Only)

In the plant city, the alarm found a specific factory called PyrB. This factory is responsible for building the bricks needed to make new DNA and RNA (the city's blueprints).

The Discovery: The authors found that when ppGpp arrives, it physically latches onto the PyrB machine.
The Effect: It's like a heavy hand slamming the brakes on the assembly line. The paper shows that adding just 200 μM of ppGpp slows down the machine significantly.
The Proof: They didn't just guess; they purified the PyrB protein and watched it melt. Normally, it holds its shape up to a certain heat, but with ppGpp, it fell apart 17 °C earlier. They also used a digital map (docking analysis) to show ppGpp fits right into the machine's "carbonyl phosphate" pocket, blocking the work.
The Catch: This brake was found in plants (Arabidopsis thaliana). In the cyanobacteria they tested, this specific factory didn't seem to react the same way. The alarm might be jamming a different door there.

2. The Energy Storage Depot (Cyanobacteria Only)

In the cyanobacterial city, there's a warehouse for storing energy in the form of glycogen (a sugar stash). The machine that fills this warehouse is called GlgC.

The Discovery: Unlike the plant factory, the ppGpp alarm doesn't stop this machine; it revs it up.
The Effect: When the alarm goes off, the GlgC machine works faster, pumping more glycogen into storage.
The Proof: The authors purified the cyanobacterial GlgC enzyme and measured its speed. With 200 μM of ppGpp, the machine sped up.
The Contrast: They tested the plant version of this machine (which makes starch instead of glycogen), and the alarm did nothing to it. The plant starch factory ignores the alarm, while the cyanobacterial glycogen factory gets a boost.

3. The CO₂ Capture Pods (Cyanobacteria Only)

Cyanobacteria have tiny, spherical structures called carboxysomes. Think of these as specialized tents where the city captures carbon dioxide to turn into food.

The Discovery: When the alarm rings, these tents start acting weird. They stop being neatly spaced out and begin to clump together in big, messy piles.
The Effect: The paper suggests this clumping is a direct result of the alarm. They saw that the alarm changes the stability of a key tent pole protein called CcmL, making it more stable (it melts at a higher temperature, jumping from 78 °C to 90 °C).
The Mystery: They tried to see if the alarm was messing with the "GPS system" (called McdAB) that usually keeps the tents spaced out. They found that the GPS was still working and still attached to the tents, even when they were clumped. This suggests the alarm isn't breaking the GPS; it's doing something else to the tents themselves to make them stick together.
The Result: The city stops growing, and the green pigment (chlorophyll) fades by 50% after 24 hours, but the cells don't die. They just hunker down.

4. The Power Plant (Cyanobacteria Only)

The city has a power cycle called the TCA cycle. One engine in this cycle is Citrate Synthase (GltA).

The Discovery: In one type of cyanobacteria (Syn6803), the alarm actually activates this engine.
The Effect: The engine runs slightly faster, and the shape of the engine changes (it shifts from a hexagon shape to a double shape).
The Caveat: This only happened in Syn6803. In the other type they tested (Syn7942), the engine didn't react at all. The paper notes that Syn7942 engines are known to form weird fractal shapes, so maybe they just behave differently.

What the Alarm Didn't Do

It's just as important to know what the alarm didn't touch.

No Direct Brake on the "PII" Protein: Scientists thought the alarm might directly talk to a protein called PII (which senses nitrogen levels). The authors tested this by mixing them together, and nothing happened. The alarm doesn't shake hands with PII directly; if PII changes, it's likely because of a chain reaction, not a direct hit.
No Direct Brake on the "RNA Polymerase" in Plants: In bacteria like E. coli, the alarm jams the main transcription machine (RNA polymerase). But in plants, the authors did not detect any change in the stability of the plant's version of this machine. The plant's transcription machine seems to ignore the alarm, or at least, the alarm doesn't stick to it in the way it does in bacteria.

The Bottom Line

This paper is like a massive "Who's Who" list for the ppGpp alarm in photosynthetic organisms. It proves that while the alarm shares some old friends with bacteria (like the translation machines), it has also found new, unique targets in plants and cyanobacteria.

In plants, it slows down DNA brick-making.
In cyanobacteria, it speeds up sugar storage and clumps the CO₂ capture tents.

The authors are careful to say this is a "foundation" for future research. They didn't solve the whole mystery of how these organisms survive stress, but they've handed us a very detailed map of the first few blocks of the city where the alarm rings.

Problem Statement
The stringent response, mediated by the signaling molecule guanosine-3',5'-bisdiphosphate (ppGpp), is a critical global regulatory mechanism allowing bacteria to adapt to stress, such as nutrient starvation. While extensively characterized in heterotrophic bacteria like Escherichia coli, the molecular mechanisms and direct regulatory targets of ppGpp in photosynthetic organisms (cyanobacteria and plant chloroplasts) remain poorly understood. Although ppGpp synthesis pathways are known in these organisms—triggered by darkness, nitrogen limitation, and various abiotic stresses—the specific protein interactors and the resulting metabolic reprogramming have not been mapped on a large scale. Existing knowledge relies heavily on indirect phenotypic observations or studies in heterotrophs, leaving a gap in understanding how ppGpp coordinates stress adaptation in photosynthetic systems.

Methodology
To address this gap, the authors employed a high-throughput chemoproteomic approach known as Proteome Integral Solubility Alteration (PISA). This method detects ligand-protein interactions by measuring changes in protein thermal stability upon ligand binding, without requiring prior knowledge of interaction partners or specific ligand design.

Organisms Studied: The study screened cell extracts from Synechocystis sp. PCC 6803 (using a spoT mutant to prevent ppGpp degradation), Synechococcus sp. PCC 7942, Arabidopsis thaliana chloroplasts, and E. coli (as a validation control).
Experimental Design: Crude proteomes were desalted to remove endogenous metabolites and incubated with varying concentrations of ppGpp (0, 200, 600, and 1000 μM). Samples were subjected to a heat gradient (40–60°C) to induce partial denaturation. Soluble proteins were recovered via ultracentrifugation, digested, and analyzed via LC-MS to quantify shifts in thermal stability (melting temperature, $T_m$ ).
Validation: Putative targets identified by PISA were further characterized using recombinant protein expression, Nano Differential Scanning Fluorimetry (NanoDSF), kinetic assays, limited proteolysis coupled with mass spectrometry, molecular docking, Isothermal Titration Calorimetry (ITC), mass photometry, and in vivo fluorescence microscopy and transmission electron microscopy (TEM).

Key Results
The study identified both conserved and novel ppGpp targets, revealing distinct regulatory strategies in plants versus cyanobacteria:

Conserved Targets: As in E. coli, ppGpp affected the stability of GTPases involved in translation (e.g., Der, Era, Obg), translation factors, and the Signal Recognition Particle (SRP) complex across all organisms. RNA polymerase subunits were also affected in cyanobacteria and E. coli, though not in A. thaliana chloroplasts, likely due to structural differences in the plastid-encoded RNA polymerase (PEP) complex.
Plant-Specific Regulation (Pyrimidine Metabolism): In A. thaliana chloroplasts, ppGpp directly binds to and inhibits Aspartate Transcarbamoylase (PyrB), the enzyme catalyzing the first step of de novo pyrimidine synthesis. NanoDSF confirmed a 17°C reduction in PyrB melting temperature upon ppGpp binding. Kinetic assays demonstrated inhibition of PyrB activity, and structural mapping suggested ppGpp binds in the carbamoyl phosphate pocket, similar to the plant-specific feedback inhibitor UMP.
Cyanobacterial Regulation (Glycogen Synthesis): In contrast to plants, ppGpp activates Glucose-1-phosphate Adenylyltransferase (GlgC) in Synechocystis sp. PCC 6803, a rate-limiting enzyme in glycogen synthesis. Recombinant GlgC showed increased activity and a shift in oligomerization state (from hexamers to dimers) in the presence of ppGpp. This activation was not observed in plant starch synthesis enzymes.
Cyanobacterial Regulation (TCA Cycle): ppGpp was found to activate Citrate Synthase (GltA) in Synechocystis sp. PCC 6803, increasing its thermal stability and enzymatic activity. This effect was not observed in Synechococcus sp. PCC 7942, consistent with known differences in their citrate synthase regulation.
Carboxysome Aggregation: PISA screening identified multiple carboxysome shell proteins (e.g., CcmL) with altered stability in Synechocystis. In vivo experiments in Synechococcus and Synechocystis strains overproducing ppGpp revealed that carboxysomes aggregate into large foci, disrupting their equidistant spacing. This aggregation occurred without cell death and was distinct from the effects of McdAB positioning system mutants, suggesting ppGpp directly modulates carboxysome spatial organization, potentially linking stress response to CO₂ fixation machinery.
Nitrogen Metabolism: While the PII protein (GlnB) showed altered stability in PISA screens, direct binding assays (ITC) indicated no direct interaction between ppGpp and GlnB, suggesting that ppGpp's effect on nitrogen regulation is indirect, likely mediated through changes in 2-oxoglutarate levels.

Significance and Claims
The paper claims to provide the first large-scale identification of direct ppGpp-binding proteins in photosynthetic organisms. By expanding the known ppGpp regulatory network, the study offers a mechanistic foundation for understanding how these organisms acclimate to nutrient and environmental stresses.

Novelty: The identification of direct inhibition of plant PyrB and activation of cyanobacterial GlgC and GltA represents a significant expansion of the known ppGpp interactome.
Mechanistic Insight: The findings clarify how ppGpp coordinates metabolic shifts, such as halting nucleotide synthesis in plants while promoting glycogen storage in cyanobacteria during stress.
Structural Organization: The discovery that ppGpp induces carboxysome aggregation provides a new perspective on the spatial regulation of carbon fixation under stress conditions.
Methodological Utility: The study demonstrates the efficacy of PISA for probing metabolite-protein interactions in complex photosynthetic proteomes, offering a template for future target deconvolution in plant and cyanobacterial biology.

The authors conclude that these findings offer a basis for future research into stress adaptation mechanisms and potential biotechnological applications, such as engineering ppGpp levels to improve crop tolerance to nutrient limitation or high light stress, and developing antimicrobial strategies targeting the stringent response.