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Bacteria and the viruses that attack them, known as bacteriophages or phages, are locked in a constant war. To survive these attacks, bacteria have built complex immune systems. Some defenses are very famous, such as CRISPR-Cas, which acts like a memory bank to remember past infections. Other systems, called restriction-modification, cut up foreign DNA to stop it. Most of these defenses rely on complicated networks of many proteins and signaling molecules to detect a threat and fight back. However, a new study reveals a defense that is surprisingly direct and simple. Instead of using a long chain of signals, a single small protein can find the danger and destroy it all by itself.
This research started when scientists studied a prophage. A prophage is a viral genome that has become permanently part of a bacterium's DNA. Scientists found this specific prophage in Pseudomonas aeruginosa. It contained a gene that made a protein no one had studied before. When researchers turned on this gene in the lab bacterium Escherichia coli, it gave strong protection against many different types of phages. This included the well-known T4 phage. The new protein was named Rip1, which stands for Ring Interacting Pore 1.
Scientists performed many experiments to find exactly which parts of the virus Rip1 attacks. Rip1 does not attack just any part of the virus. It is designed to recognize two specific structures called the portal ring and the small terminase ring. These are ring-shaped groups made of many smaller pieces. They are essential for building the phage. The portal ring makes a special spot in the virus shell where DNA is put inside. The small terminase enzyme helps load that DNA. Rip1 uses these viral rings as a guide. When Rip1 finds a portal or terminase ring from an attacking virus, many Rip1 proteins gather around it. They form a new, larger ring made only of Rip1 pieces. This assembly then sticks into the inner membrane of the bacterial cell. It creates a permanent hole, or pore, in that membrane.