OK, you may think, phages are great, but the viruses that infect us are certainly not cool. And many bacterial attack systems, such as toxins and the genetic guns used to inject them into cells, as well as the camouflage they use to evade the immune system, are known or suspected to have phage origins. ![]() Beautiful detective work has shown that a bacterial gene needed for cell division also arose through “domestication” of a phage toxin gene. It’s not just bacterial switches that appear to be phage inventions. Miquel Sánchez-Osuna/Created with, CC BY-NC-ND Over time, the bacterium may adapt the phage’s switch to control its own SOS response genes. This means that when DNA damage occurs, the phage won’t be able to reform itself and burst out. But once the phage’s DNA is part of the bacterium’s, mutations can disrupt the phage’s genetic material and render it inactive. When a temperate phage infects a bacterial cell and integrates its genome with the cell’s DNA, it typically lays dormant until it’s triggered to burst out of the cell. This suggests that bacterial SOS switches are in fact phage switches that got retooled eons ago. In our recent report, we discovered that the SOS response of Bacteroidetes, a group of bacteria that comprise up to a half of the bacteria living in your gut, is under control of a phage switch that was retooled to implement the bacteria’s own complex genetic programs. Our previous research and work by other researchers indicates that phages got there first. This prompts the question: Who invented the switch, bacteria or viruses? Perhaps not surprisingly, bacterial and phage switches are evolutionarily related. Bacteria orchestrate the SOS response using a switch-like protein that responds to DNA damage: It turns on if there is damage and stays off if there isn’t. This is known as the bacterial SOS response because, if it fails, the cell is toast. If it is, they activate a set of genes that attempt to repair the DNA. CNX OpenStax/Wikimedia Commons, CC BYīacteria have retooled the mechanisms controlling that life cycle to generate a complex genetic system that my collaborators and I have been studying for over two decades.īacterial cells are also interested in knowing if their DNA is getting busted. Temperate phages, on the other hand, follow the lysogenic cycle and stay dormant inside their host’s DNA until they’re triggered to burst out. Virulent phages follow the lytic cycle of viral reproduction, destroying their hosts as soon as they complete replication. ![]() The genes that direct phages to replicate and burst out are turned off unless DNA damage is detected. If the cell’s DNA is being damaged, that means the DNA of the resident phage is likely to go next, so the phage wisely decides to jump ship. It’s sort of a “Houston, we have a problem” signal. Many temperate phages use DNA damage as their trigger. Then they revert to virulent behavior: replicate and burst out. They fuse their DNA with the cell’s and may lay dormant for years until something triggers their activation. Temperate phages, on the other hand, play the long game. They enter the cell, hijack its components, make copies of themselves and burst out. Virulent phages, like many other viruses, operate on an invade-replicate-kill program. Phages come in two main flavors: temperate and virulent. To see how this plays out, let’s take a deeper look at the phage life cycle. This means that phages have room to carry extra genetic baggage: genes that are not actually necessary for the phage’s survival that it can modify at will. In many cases, the shell has more space than the phage needs to store the DNA essential for its replication. Most phages have a rigid shell called a capsid that is filled with their genetic material. Like all viruses, phages also have high replication and mutation rates, meaning they form many variants with different characteristics each time they reproduce. Phages are the most abundant life form on the planet, with a nonillion – that’s a 1 with 31 zeroes after it – of them floating around the world at any moment. Kristina Dukart/iStock via Getty Images Plus ![]() Bacteriophage caspids can carry extra DNA that the virus can tinker with.
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