Shedding light on YfhS and YjlC: novel effectors of the NADH dehydrogenase activity of the electron transport chain in Bacillus subtilis

This study identifies YfhS and YjlC as novel regulatory partners that form a functional complex with the NADH dehydrogenase Ndh in *Bacillus subtilis*, revealing a critical mechanism for moderating NADH levels and preventing lethality that offers potential targets for new antibiotics against resistant bacteria.

The Gist

The Big Picture: A Power Plant in a Bacterial City

Imagine the bacterium Bacillus subtilis as a bustling city. To keep the lights on and the factories running, this city needs energy. It generates this energy through a process called respiration, which is like a massive power plant.

Inside this power plant, there is a critical machine called the Electron Transport Chain (ETC). Think of this chain as a conveyor belt that moves high-energy "packages" (electrons) from one station to the next to generate electricity (ATP).

The first station on this conveyor belt is the NADH Dehydrogenase (Ndh). Its job is to take a fuel source called NADH, strip it of its electrons, and hand those electrons over to the belt. This is a vital job, but it's also dangerous. If the belt moves too fast or gets jammed, it sparks and creates "fire" (Reactive Oxygen Species), which can burn down the city.

The Problem: The City Without a Safety Valve

The scientists in this study were investigating a mysterious protein called YfhS. They noticed that when they removed YfhS from the bacteria, the city fell apart. The bacteria grew very slowly and looked deformed (like tiny, curved squiggles instead of healthy rods).

It was as if the city's power plant was running wild, but no one knew why.

The Discovery: Finding the Saboteurs

To figure out what YfhS did, the scientists waited. Eventually, some of the sick bacteria spontaneously "cured" themselves. They started growing into normal-sized colonies again. The scientists sequenced the DNA of these "cured" bacteria and found a clue: they all had a tiny mutation (a typo) in the gene for the Ndh machine.

The Analogy: Imagine the Ndh machine is a water pump. In the sick bacteria (without YfhS), the pump was spinning so fast it was sucking the city dry. The "cured" bacteria had a broken pump that spun a little slower, which saved them.

This told the scientists that YfhS is a regulator. It doesn't build the machine; it acts like a brake or a governor to make sure the Ndh machine doesn't spin out of control.

The New Team: YfhS, YjlC, and Ndh

The study revealed a fascinating new team dynamic:

1. Ndh: The main engine that processes the fuel.
2. YjlC: A new partner that acts like the anchor or the chassis that holds Ndh in place. You can't have a working engine without the chassis.
3. YfhS: The traffic cop or moderator.

The scientists found that YfhS physically touches YjlC. When YfhS is present, it keeps the Ndh engine running at a safe, steady speed. When YfhS is missing, the engine revs too high, burning up all the fuel (NADH) too quickly.

The "Overdose" Experiment:
The scientists tried to force the bacteria to make more of the Ndh engine.

• In a normal city, making more engines was fine.
• In a city without YfhS (the traffic cop), making more engines was lethal. The engines spun so fast they destroyed the cell.

This proved that YfhS is essential to prevent the power plant from overheating.

The Bigger Mystery: The "ResDE" Radio

The scientists also discovered that YfhS talks to another system in the cell called ResDE. Think of ResDE as the city's central radio station that broadcasts instructions on how fast the power plant should run based on the weather (oxygen levels).

It seems YfhS acts as a signal booster for this radio. Without YfhS, the radio signal gets garbled, and the power plant doesn't get the right instructions, leading to chaos.

Why Should We Care?

This isn't just about bacteria; it's about medicine.

1. Human Safety: Humans have a similar power plant, but we use a different version of the Ndh machine (Type I). Bacteria use a Type II machine. Because humans don't have the Type II machine (or the YfhS/YjlC partners), drugs that target these specific bacterial proteins would kill the bacteria without hurting us.
2. New Weapons: Since antibiotic resistance is a huge problem, finding new "weak spots" in bacteria is crucial. YfhS and YjlC are new, promising targets for designing the next generation of antibiotics.

Summary in One Sentence

The bacteria need a protein named YfhS to act as a traffic cop for its energy machine; without YfhS, the machine spins too fast, burns out the cell's fuel, and causes the bacteria to die, but if we can target this traffic cop with drugs, we might be able to kill dangerous bacteria without hurting humans.

Technical Summary

1. Problem Statement

Oxidative phosphorylation is the primary energy generation mechanism in respiring cells, but the electron transport chain (ETC) is a major source of reactive oxygen species (ROS). Consequently, the production of high-energy electrons must be tightly regulated. In Bacillus subtilis, the Type II NADH dehydrogenase (Ndh, encoded by ndh) is the primary enzyme oxidizing NADH to NAD+ to feed electrons into the ETC.

• Known Regulation: The transcription factor Rex monitors intracellular NADH levels to regulate the yjlC-ndh operon. High NADH relieves Rex repression, increasing Ndh production.
• The Gap: Previous work from the authors' lab identified that deletion of yfhS (a small, conserved protein of unknown function) results in severe growth defects and altered cell morphology. While ndh mutations could suppress these defects, the mechanistic relationship between YfhS, the Ndh enzyme, and its partner YjlC was unknown. Furthermore, the functional composition of the Type II NADH dehydrogenase complex in B. subtilis had not been experimentally validated as a multi-protein complex.

2. Methodology

The study employed a combination of genetic, genomic, and biochemical approaches:

• Phenotypic Screening & Suppressor Analysis: The authors isolated spontaneous "Large Colony Isolates" (LCIs) from a ΔyfhS background that exhibited restored growth. Whole-genome sequencing (WGS) was performed to identify suppressor mutations.
• Strain Construction: Generation of single, double, and triple deletion mutants (ΔyfhS, Δndh, ΔyjlC, Δrex, ΔresD, ΔresE) and strains with inducible expression of yjlC, ndh, or the yjlC-ndh operon (using an IPTG-inducible promoter, $P_{IPTG}$) to uncouple expression from native Rex/ResDE regulation.
• Microscopy: High-resolution fluorescence microscopy (FM4-64 staining) was used to quantify cell cross-sectional area and morphology.
• Transcriptional Reporting: A bacterial luciferase reporter ($P_{yjlC}$-lux) was used to monitor the transcriptional activity of the yjlC-ndh operon as a proxy for intracellular NADH/NAD+ ratios.
• Protein-Protein Interaction: Bacterial two-hybrid (B2H) assays were conducted in E. coli to test for direct physical interactions between YfhS, YjlC, and Ndh.
• Genetic Interaction Analysis: Epistasis analysis was performed by combining deletions (e.g., ΔyfhS ΔresE) to map regulatory pathways.

3. Key Contributions

1. Discovery of a Novel Regulatory Mechanism: Identification of YfhS as a critical post-transcriptional moderator of Ndh activity, independent of the Rex transcription factor.
2. Definition of the Ndh Complex: Providing the first experimental evidence that the functional Type II NADH dehydrogenase in B. subtilis is a two-protein complex consisting of the enzyme Ndh and the membrane-associated protein YjlC (a DUF1641 domain protein).
3. Elucidation of YfhS Function: Demonstrating that YfhS prevents the lethal over-oxidation of NADH (depletion of NADH/accumulation of NAD+) by dampening the activity of the YjlC-Ndh complex.
4. Link to ResDE Signaling: Proposing a model where YfhS interacts with YjlC to modulate the ResDE two-component system, thereby fine-tuning the expression of the yjlC-ndh operon in response to metabolic state.

4. Key Results

• Suppression of ΔyfhS Phenotypes:

  • ΔyfhS strains form small colonies and have curved, reduced cell surface areas.
  • Spontaneous LCIs that restored growth were found to harbor missense mutations in ndh (specifically G9A and S109F, located in the FAD cofactor binding pocket).
  • Crucially, a double deletion (ΔyfhS Δndh) did not restore growth, indicating the suppressor mutations do not simply abolish Ndh function but likely reduce its catalytic efficiency to a tolerable level.

• Lethality of Complex Overexpression:

  • Overexpression of ndh or yjlC individually was not toxic.
  • However, simultaneous overexpression of the yjlC-ndh operon in a ΔyfhS background was lethal, causing severe cell shrinkage. This suggests YfhS is required to prevent hyperactivity of the YjlC-Ndh complex.

• Transcriptional Analysis (NADH/NAD+ Ratios):

  • The P_{yjlC}-lux reporter showed elevated activity in ΔyfhS strains, suggesting high intracellular NADH levels (which normally derepress Rex).
  • However, when yjlC-ndh was overexpressed in ΔyfhS, luminescence dropped drastically, and cells died. This indicates that without YfhS, the complex rapidly depletes NADH, leading to a toxic accumulation of NAD+ and a collapse in the redox balance.

• Protein Interactions:

  • Bacterial two-hybrid assays confirmed direct physical interactions between YjlC-Ndh, YjlC-YfhS, and self-interactions of all three proteins.
  • This confirms YjlC acts as a membrane anchor for Ndh and that YfhS is a direct binding partner.

• Regulatory Pathway (ResDE):

  • Deletion of resD (part of the ResDE two-component system) resulted in a small colony phenotype similar to ΔyfhS.
  • Crucially, the double deletion ΔyfhS ΔresE restored wild-type growth and colony size.
  • This implies YfhS normally helps maintain ResE in a state that allows ResD to repress yjlC-ndh. Without YfhS, ResD repression is compromised, leading to uncontrolled expression and activity of the complex.

5. Significance

• Fundamental Biology: The study redefines the understanding of bacterial respiration by identifying YfhS and YjlC as essential, previously uncharacterized components of the NADH dehydrogenase machinery. It reveals a complex regulatory layer involving protein-protein interactions and the ResDE system that operates alongside the well-known Rex transcriptional control.
• Redox Homeostasis: It highlights the critical importance of maintaining a precise NADH/NAD+ ratio. The study demonstrates that while transcriptional upregulation of Ndh is a standard response to high NADH, unmodulated enzymatic activity (without YfhS) is lethal due to NAD+ toxicity.
• Antibiotic Development: Type II NADH dehydrogenases are absent in human mitochondria, making them attractive targets for new antibiotics. The identification of YfhS and YjlC as essential regulators of this complex in B. subtilis (and likely other Firmicutes) expands the pool of potential drug targets. Disrupting the YfhS-YjlC-Ndh interaction could selectively kill pathogenic bacteria by disrupting their redox balance.
• Biotechnology: Understanding these regulatory nodes could aid in engineering B. subtilis for industrial fermentation, where optimizing NADH/NAD+ ratios is crucial for maximizing the yield of metabolic byproducts.