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The evolution of radio resistance is more complicated than previously thought



The evolution of radio resistance is more complicated than previously thought

Induced radio-resistant E. coli evolves complex mutation profiles as experimental evolution continues and the level of radio-resistance increases. Credit: Michael M. Cox and co-authors

The hardiest organisms on Earth, called extremophiles, can survive extreme conditions such as extreme dryness (desiccation), extreme cold, space vacuum, acid, or even high-level radiation. So far, the toughest of all seems to be the bacterium Deinococcus radiodurans, capable of surviving radiation doses a thousand times greater than those deadly to humans. But until now, scientists have been puzzled as to how radio resistance could have evolved in different organisms on our planet, naturally protected from solar radiation by its magnetic field. While some scientists suggest that radio resistance in extremophilic organisms may have evolved alongside other types of resistance, such as resistance to desiccation, the question remained: Which genes are specifically involved in radio resistance?

To answer this question, Dr. Cox̵

7;s team at the University of Wisconsin-Madison decided to “let the cells tell them.” The researchers started with the naturally non-resistant bacteria, E. coli, and exposed it to iterative cycles of high-level irradiation. After many cycles of radiation exposure and overgrowth, some radio-resistant populations have emerged. Using whole genome sequencing, the researchers studied the genetic alterations present in each radio-resistant population and determined which mutation provided radio-resistance to the bacteria.

In their first study, Dr. Cox’s team began by exposing E. coli to 50 ionization cycles (Bruckbauer et al 2019b). After about 10 rounds, some radio-resistant populations emerged, and after 50, studying their genetic profile revealed three mutations responsible for radio-resistance, all in genes related to DNA repair mechanisms. Here, in their new study, the team exposed the bacteria to another 50 cycles of radiation exposure and selection.

The results published in Frontiers in microbiology show that populations of radioresistant E. coli continued to evolve and subpopulations emerged. Surprisingly, while the radio resistance induced by the first ionization series could be mainly associated with three mutations, the second induced hundreds of mutations including large deletions and duplications of several genes. “The four populations that we are evolving in this new study have now reached levels of radio-resistance that are approaching the levels seen with Deinococcus radiodurans. As the current trial progresses, the genomic alterations have proved much more complex than expected.” . Says Dr. Cox.

Although this time it is more difficult to identify all the mutations contributing to the increase in radio resistance, the researchers show that multiple cellular metabolisms are involved (ATP synthesis, iron-sulfur cluster biogenesis, cadaverine synthesis and response of reactive species of ‘oxygen). Furthermore, this study demonstrates that radio resistance can develop to the level of Deinococcus radiodurans, regardless of resistance to drying. As radiation exposure and experimental evolution continue, more data is being collected on how to induce radio resistance in bacteria. This could one day serve as a valuable mutation toolbox for designing radioresistant probiotics by helping patients treated with radiation therapy or astronauts exposed to space radiation, for example.


THE. Radiation resistant coli developed in the laboratory allows you to visualize DNA repair


More information:
Frontiers in microbiology, DOI: 10.3389 / fmicb.2020.582590, www.frontiersin.org/articles/1… 2020.582590 / abstract

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The evolution of radio resistance is more complicated than previously thought (2020, September 22)
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