A growth-inhibitory dUTPase from Mosigvirus AV110 disrupts cell division and host physiology in Escherichia coli

Inside every living cell, a constant, quiet battle rages over the building blocks of life. To copy its genetic code, a cell needs a steady supply of specific chemical parts, including one called dUTP. If this part slips into the genetic code by mistake, it causes errors that can break the cell's instructions. To prevent this, cells use a specialized tool, an enzyme, to break down the excess dUTP before it can cause harm. This tool is found in almost all forms of life, from humans to bacteria. However, some viruses that infect bacteria, known as phages, have stolen or invented their own versions of this tool. These viral tools often have a dual purpose: they help the virus build its own genetic material, but they also disrupt the host cell's supply of these essential parts, effectively hijacking the cell's machinery to serve the virus instead.

A team of researchers in Denmark and Belgium recently turned their attention to a specific viral tool found in a phage called Mosigvirus AV110. This virus infects a type of bacteria known as Escherichia coli, specifically strains that are resistant to many common antibiotics. The researchers wanted to understand exactly what happens when this viral tool, a protein named Gp135, is active inside the bacterial cell. They were particularly interested in how the presence of this single protein changes the shape of the bacteria, stops them from dividing, and ultimately kills them. By studying this interaction, the scientists hoped to learn more about how viruses manipulate their hosts and to map out the vast, hidden diversity of these tools across the viral world.

The researchers began by introducing the gene for the Gp135 protein into several different strains of the antibiotic-resistant bacteria. They then watched what happened as the bacteria started to produce the protein. The results were immediate and dramatic. In the presence of the protein, the bacteria stopped growing. When the scientists measured the density of the bacterial culture, they saw that the number of living cells dropped significantly, with some strains showing a reduction of up to four orders of magnitude. This meant that for every ten thousand bacteria that should have been there, only a handful remained. The effect was not limited to a single strain; it worked across a wide variety of the bacteria tested, suggesting that the protein targets a fundamental process that all these cells share.

To see the physical changes happening inside the cells, the team used powerful microscopes to watch the bacteria over time. In the beginning, the cells looked normal. But as the protein continued to be produced, the bacteria began to stretch out, becoming long, thin filaments instead of their usual short, rod-like shape. They stopped dividing. Normally, a bacterium grows and then splits in half to create two new cells, but these cells kept growing longer without ever splitting. The researchers also stained the cells to look at their internal structures. They found that the DNA inside the cells was not replicating. In healthy bacteria, the genetic material copies itself and moves to opposite ends of the cell before division. Here, the DNA signal was weak and did not spread out, indicating that the copying process had stalled.

As the hours passed, the damage became more severe. The researchers observed strange, bright bands appearing along the length of the stretched cells. These bands were not part of the cell wall or the membrane; they were empty spaces, or voids, where the inside of the cell had pulled away from the walls. The cells were essentially hollowing out. Eventually, the cell membranes broke down, and the bacteria died. The scientists noted that this pattern of death, where cells stop dividing, stretch out, and develop internal gaps, is similar to what happens when bacteria are starved of a specific nutrient needed to build DNA. In this case, the viral protein was likely causing a shortage of the building blocks the bacteria needed to copy their genetic code, leading to a collapse of the cell's internal structure.

The team then looked closely at the protein itself to understand how it worked. They found that the Gp135 protein is very similar in shape and structure to a well-known tool found in another famous virus, the T4 phage. This T4 tool is known for its ability to break down not just dUTP, but also a related molecule called dCTP. The researchers' analysis suggested that the Gp135 protein acts in the same way. By breaking down these essential building blocks, the protein depletes the cell's supply, preventing the bacteria from copying its DNA. Without new DNA, the cell cannot divide. The researchers also found that this protein helps the virus create special, modified versions of its own genetic material, which act as a shield against the bacteria's natural defense systems.

Finally, the researchers took a broader view to see how common this type of viral tool is. They gathered genetic sequences from hundreds of different viruses and built a family tree to see how they are related. They discovered that these tools are not all the same. While most known tools form a three-part structure, the ones found in the Mosigvirus and similar viruses form a two-part structure. Even more interestingly, they found that these two-part tools fall into two distinct groups that are not closely related to each other, suggesting they evolved separately. There is also a fourth, mysterious group of tools that scientists have not yet studied, which appear to be very diverse and could have different functions entirely.

The study concludes that the Gp135 protein is a powerful weapon in the viral arsenal. By disrupting the supply of genetic building blocks, it halts the host cell's ability to reproduce, leading to cell death and allowing the virus to take over. This research highlights the complex and varied ways viruses have evolved to manipulate their hosts. It also reveals that there is still much to learn about the different types of these tools that exist in nature, with entire groups of them waiting to be understood. The findings suggest that the ability to hijack a cell's chemistry is a widespread and successful strategy for viruses, one that relies on a diverse and evolving set of molecular tools.