Meet Deinococcus radiodurans, one of the world’s toughest bacterium. It’s an extremophile and one of the most radiation-resistant organisms known on Earth. This hardy little bacterium can survive over a thousand times the amount of radiation that would kill a human. Its tolerance to many harsh conditions has earned itself the name “Conan the Bacterium,” after the movie Conan the Barbarian. D. radiodurans is a spherical bacterium and four individual cells are typically stuck together forming a square shaped cluster. Aside from radiation resistance, D. radiodurans is also resistant to ultraviolet light and desiccation.
When hit with radiation, DNA molecules break apart. The more radiation, the more the DNA falls apart. D. radiodurans’s radiation resistance is due to the immense ability of the bacterium to repair its DNA after damage. It can withstand enough radiation to break its DNA hundreds of times and pieces everything back together in merely hours.
Cells repair DNA in a process called homologous recombination in which broken DNA pieces are joined together in their original arrangement. Broken fragments of DNA are aligned to an intact fragment based on identical DNA sequence (thus the name homologous). This homologous piece of DNA serves as a template for repair to bring the broken pieces back together. As an analogy, let’s think of the genome as a book with the sentences as the DNA. You have two copies of the book and in each book, 20-page chunks are ripped out at random. To put the pieces back together, you compare the beginnings and ends of the 20-page segments from each book to the other to look for pages that are identical between the two books.
Now homologous recombination is a process that is widespread across all domains of life. But D. radiodurans is unique in that it has many more copies of its chromosome than the typical bacteria. D. radiodurans has four copies of its genome whereas E. coli only has one. The extra copies help increase the chances that any broken DNA fragments will match to homologous sequences, making successful homologous recombination easier to achieve. It is not just the multitudes of DNA that helps D. radiodurans withstand radiation. A cell that loses its protein function will die regardless of how much DNA it has. Radiation can also inactivate proteins through oxidative stress but D. radiodurans has multiple ways to protect their proteins from oxidative stress.
You may be wondering why something so resistant to radiation exists on Earth. There aren’t any places on Earth where bacteria would have been exposed to that much radiation. D. radioduran’s resistance to radiation may make you wonder whether it originated in space. However, D. radiodurans is genetically similar to the thermophile (heat-loving) Thermus thermophilus. Because growth at high temperatures also damage proteins, it is possible that the multiple resistances of D. radiodurans originated from heat resistance.
