Imagine if, every time you replaced a cracked brick in your house, you could break it down and reuse the pieces to build a new wall. That’s essentially what many bacteria, such as Agrobacterium tumefaciens, do every day. They don't deal with bricks, but with their cell walls. For these bacteria, it’s a survival strategy that involves antibiotic resistance and maintaining the shape critical for survival. To understand this fascinating and critical process, let’s dive into the secrets of peptidoglycan recycling together!
Meet Agrobacterium tumefaciens: Plant Hacker Extraordinaire
Before diving into the world of recycling, let's meet the show's star! Agrobacterium tumefaciens is best known as nature’s genetic engineer. It can sneak its own DNA into plant cells, causing tumors (also known as crown galls, Figure 1.), and hijack the plant’s metabolism to serve its own needs. Scientists have repurposed this trick and use A. tumefaciens as a gene delivery system for genetically engineered crops. As a key material in plant genetic engineering, it is crucial to understand the critical mechanisms and regulation of A. tumefaciens' function to optimize its use in biotechnology. This knowledge could help researchers further develop better methods of genetic editing with A. tumefaciens or improve the bacterium’s performance. This could enhance its stability and operational efficiency, potentially leading to higher transformation success rates in targeted plant species.
Figure 1, a crown gall produced by a plant that is infected with A. tumefaciens, from the University of Minnesota.
Like all bacteria, A. tumefaciens must counteract osmotic pressure and maintain cell shape and integrity. It relies on a strong yet flexible outer structure made of peptidoglycan (PG) to do so (Holtje, 1998; Vollmer et al., 2008a). Imagine a dense mesh of sugar chains linked by short peptides. This structure is like the chainmail armor of the bacterial world. As A. tumefaciens grows and divides, it must constantly remake its PG layer. But what happens to the old fragments that are shed during this process? That’s where recycling comes in!
Why Recycle Peptidoglycan?
Peptidoglycan is expensive and energy-intensive to produce. Instead of discarding peptidoglycan fragments (called muropeptides), many bacteria absorb them, break them down, and reuse the pieces via the peptidoglycan recycling pathway.
In E. coli, this process uses a well-known protein called AmpG, which acts as a transporter that brings the muropeptides back into the cell. Once inside, enzymes reshape and reconfigure the pieces, which are then used to make fresh PG and control other bacterial functions, such as antibiotic resistance.
However, here’s the twist: A. tumefaciens doesn’t have AmpG!
So, how does it recycle?
Enter the YejBEF-YepA Recycling Squad
To solve this mystery, Gilmore and Cava conducted a study (2022) using the antibiotic Fosfomycin to prove that A. tumefaciens can recycle. Fosfomycin blocks the first committed step in PG synthesis. If a bacterium can recycle PG, then it can bypass this block by taking the old PG fragment and survive. Next, they performed a genome-wide transposon sequencing screen (Tn-Seq) that can identify genes essential for survival in the presence of Fosfomycin. Their results pointed to a set of genes forming an ABC transporter system: YejB, YejE, YejF, and a substrate-binding protein (SBP) that they eventually renamed YepA.
ABC (ATP-binding cassette) transporters are protein complexes that use energy from ATP to transport molecules across membranes actively. In this case, YejB and YejE are membrane-spanning proteins that form the transport channel. YejF is the ATPase that fuels the process. YepA (formerly Atu1774) is the substrate-binding protein (SBP). It floats in the space just outside the inner membrane and recognizes PG fragments (muropeptides), delivering them to the transporter.
Figure 2. The structure of ABC transporter systems: YejBEF-YepA, by Gilmore & Cava (2022).
This configuration is classic for ABC transporter systems, which typically consist of two membrane components, a binding protein, and ATPases. The presence of this structure, coupled with the fact that YejABEF has been demonstrated to import a variety of peptides and function as an ABC transporter in other bacteria (Wang et al., 2016), has led the researchers to propose that this was the missing PG importer in A. tumefaciens.
Result and Figures!
Now that we’ve learned about the YejBEF-YepA system, let’s examine Gilmore and Cava’s results closely to understand better and visualize this knowledge (2022).
Figure 3. Growth curve of A. tumefaciens strains of WT and ΔyejABEF on LB10 and LB0, by Gilmore & Cava (2022).
This graph shows how A. tumefaciens strains grow over time in either a rich medium (LB10) or a nutrient-limited medium (LB0). LB10 contains 10 grams per liter of NaCl, while LB0 contains none, creating osmotic stress and nutrient limitation. The vertical axis shows the optical density (OD) at 600 nanometers (OD600), which is a measure of how cloudy the culture is, or how densely the bacteria are growing. While the wild type and ΔyejA strains thrive in both conditions, the ΔyejABEF mutant struggles dramatically in LB0 and barely grows. Clearly, when yejBEF is removed (except yejA, which has no effect), the bacteria cannot undergo the peptidoglycan recycling process necessary for cell growth and maintenance under osmotic stress. Thus, growth after editing yejBEF is poor, especially under stress.
Figure 4. WT and ΔyejABEF A. tumefaciens cells grown on LB10 and LB0 were observed under Phase contrast microscopy, by Gilmore & Cava (2022).
Under a microscope, we can actually see the cells themselves (Figure 4.). In the absence of PG recycling, they are swollen, misshapen, and bursting, especially under LB0 conditions. Together, these cells demonstrate that, without PG recycling, the bacterial cell wall literally falls apart.
No Recycling Means No Resistance
Muropeptides serve not only as building material, but also as signals. In many bacteria, when peptidoglycan (PG) is broken down, the resulting fragments are imported and trigger antibiotic resistance pathways.
Figure 5. A: comparison between WT, ΔyejABEF, ΔampC, and ΔyejABEFΔampC strains under Ampicillin influence; B: comparison between ΔyejA and ΔyepA strains under Ampicillin influence, by Gilmore & Cava (2022).
In A. tumefaciens and a lot of other bacteria, AmpR, a transcriptional regulator, handles this signal. When muropeptides accumulate inside the cell, AmpR is activated, turning on the production of AmpC. AmpC is a β-lactamase enzyme that breaks down β-lactam antibiotics, such as penicillin. In most bacteria, muropeptides are imported via AmpG (Vadlamani et al., 2015). However, in A. tumefaciens, AmpG is absent, so YejBEF-YepA takes over this role.
How do we know this? Figure 5B shows the evidence. Researchers used ampicillin MIC test strips to compare the resistance of different bacterial strains. The ΔyejABEF mutant is dramatically more sensitive to ampicillin than the wild type and is even more sensitive than the ΔampC strain, which does not produce the β-lactamase enzyme; this might show that the A. tumefaciens resistance to antibiotics is only partially dependent on ΔampC. The double mutant (ΔyejABEF ΔampC) is also hypersensitive and more similar to the ΔyejABEF mutant.
Figure 5D highlights the importance of YepA as the substrate-binding protein (SBP). While the ΔyejA mutant shows no change in antibiotic sensitivity, the ΔyepA mutant displays marked hypersensitivity to ampicillin. This suggests that, unlike most bacteria, where YejA serves as the SBP for the YejABEF system, YepA (formerly Atu1774) plays that role in A. tumefaciens instead.
Together, YejBEF and YepA play a critical and central role in antibiotic defense.
Why This Matters?
This discovery not only fills in our knowledge of Agrobacterium tumefaciens, an important tool for plant genetic editing, but also reveals a brand new set of bacterial survival mechanisms involving proteins. YejBEF-YepA.
These proteins turned out to be central players in cell wall maintenance and β-lactam antibiotic resistance. When researchers deleted this transporter, the consequences were dramatic. Cell walls fell apart, bacteria became hypersensitive to common antibiotics like ampicillin, and growth was hindered even under mild stress conditions. In other words, losing PG recycling meant losing structural integrity, chemical and physical defenses, and a competitive survival advantage.
Interestingly, YepA is a previously overlooked protein that turned out to be the key binding component, replacing the dominant YejA in this system. YepA isn’t alone; it is present in many other Alphaproteobacteria, especially plant-associated species. This raises the exciting possibility of studying how this pathway could influence microbial communities of Alphaproteobacteria differently or similarly.
About the author:
Krista Xiao ‘25, biology major.
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