Gamma Rays Won’t Give You Powers—But This Bacterium Thrives With Them

By: Kim Beaver '25

Figure 1. The author’s creative illustration of Deinococcus radiodurans as a superhero.

Superheroes are prevalent figures in popular culture, with many of them gaining their superpowers through radiation exposure. Despite these inspiring stories of being able to shoot webs or turning green from radiation, the reality is that most humans would not be able to survive 4 Gray units (Gy) of gamma radiation. Radiation can damage DNA, the instructions to building and maintaining life, which in turn destroys cells and their corresponding tissues, making it impossible to survive with enough exposure. Therefore, it is incredible that anything can survive high radiation levels. This is why the extremophile Deinococcus radiodurans, a bacterium that can survive radiation doses 28,000 times what humans can, caught my attention. Bacteria are generally known to be “simplistic” lifeforms, having what seems to be less complex cellular structures than those of human cells. While this is true to a certain extent, it just makes bacteria even more impressive, doing more with less! I think it is important to explore these unique cases to get a better understanding of the complexities of the world around us, and in that way, gain a deeper appreciation for the diversity of life.

Figure 2. A cross section micrograph image of a D. radiodurans tetracoccus, a cluster of four cells.

In the case of D. radiodurans, it uses the molecular machinery common to many bacteria in a unique way that enhances its resistance to radiation. The lab led by Sharma et al. (2024) explored how D. radiodurans utilized different proteins involved in natural transformations to repair damaged DNA. This blog post will focus on the Sharma et al. (2024) findings on the roles of periplasmic endonuclease A (EndA) and DprA following DNA damage caused by gamma radiation or mitomycin C (MMC).

Natural transformations (NT) are a type of horizontal gene transfer in which extracellular DNA (eDNA) is incorporated into the bacterial chromosome. This process can be used for DNA repair, which is important in the context of radiation induced damage, including double-strand breaks (DSB) and single-strand breaks (SSB). EndA is a NT protein that processes eDNA into a usable single stranded DNA, which then can be transported into the cell cytoplasm. DprA is a single-stranded DNA binding protein that loads RecA onto the DNA, initiating homologous recombination and facilitating repair.

A pulse field gel-electrophoresis was used to isolate intact chromosomal DNA using an electric field over a matrix of gel. Very small, damaged DNA fragments would not appear on the gel, meaning the DNA must be repaired in order to be visible. Following exposure to MMC, the mutant that lacked EndA (ΔendA) showed no repairs to the DNA after 6 hours (Figure 3B). In comparison, the wild-type had the first signs of repairs at about 3 hours (Figure 3A). This was expected because without EndA, eDNA would not be able to be broken down to be used for repair. Mutants lacking the other NT repair proteins may not show the same effect due to redundancy in their function. Interestingly, the mutant lacking DprA (ΔdprA) had a surprising reaction to MMC by demonstrating a faster repair time than the wild type, in only 2 hours (Figure 3C).

Figure 3: Pulse-field gel electrophoresis of wild type (WT) and natural transformation gene mutants both untreated and post 10-µg/mL MMC treatment.

With ΔdprA displaying quicker DNA repair, two glaring questions emerge- why would a protein that initiates homologous recombination slow down the repair process and why have this protein in the first place?

Both single-strand annealing (SSA) and error-free DSB repair via synthesis-dependent strand annealing (ESDSA) are mechanisms of DNA repair utilized by cells. SSA is a repair pathway of DSB that involves the pairing of homologous sequences that are near each other. It is a quick process but more vulnerable to error since it might also remove regions between the homologous sequences. In contrast, ESDSA uses homologous strands as templates for repair rather than for repair themselves. Although slower, this is a more accurate and less unstable method. SSA can sometimes be a precursor for ESDSA through the generation of substrates that are processed further as a function of the latter.

DprA acts as a mediator for loading RecA onto single-stranded DNA, crucial in the amount of RecA is utilized and when. RecA is necessary for ESDSA as it searches along the DNA for homologous sequences- the correct template. In certain conditions, DprA may slow loading of RecA, making SSA the dominant pathway. This might be beneficial in higher stress situations that require quicker repair. In the case of D. radiodurans, it is believed that DprA competes with other RecA mediator proteins, slowing the ESDSA pathway in favor of SSA. In the ΔdprA mutants, the repair was noticeable after 2 hours, with high percentages of survival. Sharma et al. (2024) suggested that without DprA interference, other mediator proteins are successful in recruiting RecA, allowing ESDSA to become the dominant pathway.

Taking repair a step further, Sharma et al. (2024) then looked at cell survival following MMC, gamma radiation, and UV radiation (both alone and in combination to the others, representative of extreme damage). After gamma radiation treatment, the ΔendA population survival was comparable to the wild type post and had a smaller percentage of survival in the MMC treatments (Figure 4). This result was expected as more cells would struggle to survive without EndA repairing DSBs. In contrast, consistent with the gel electrophoresis findings, the ΔdprA had slightly higher survival percentages than the wild type across treatments, with the exception of only UV radiation (Figure 4). The ΔendA ΔdprA double mutant had outstanding survival throughout damaging treatment, another finding that is counterintuitive.

Figure 4. Percentages of different mutants or wild type cell survival following exposure to 6-kGy doses of gamma radiation, 1,500-J/m² UV radiation, or both, and (B) MMC (20 µg/mL), 1,500-J/m² UV radiation, or both.

The difference in cell survival between the ΔendA ΔdprA double mutant and either the ΔendA or ΔdprA single mutants bring to light a potential interaction between the two proteins. DprA might play a more regulatory role in the pathway EndA is involved in. Sharma et al. (2024) suggests that the higher survival of the double mutant may be due to the distinct repair pathways that each protein contributes to. It seems that under extreme stress, it is most beneficial to have neither NT protein, however, I wonder how survival of the bacteria may differ under less stressful conditions or in response to different types of DSBs. Although an interesting perspective, more research is needed in the specifics of the role of EndA for DNA repair.

Although SSA and ESDSA are separate repair mechanisms, they can interact with SSA preparing DNA for the latter. The ability of D. radiodurans to utilize these two pathways to balance speed and efficiency contributes to its resistance to radiation. The findings of Sharma et al. (2024) broaden our understanding of other radiation resistant bacteria. These organisms may have similar and distinct pathways that exhibit the same type of interacting elements. The complexities of intracellular interactions within D. radiodurans are yet to be fully understood, but this field of research fosters a deeper appreciation for the evolutionary diversity that allows some bacteria to live in extremely stressful environments.


About the Author:

Kim Beaver '25 is a graduating senior of Mount Holyoke College, double majoring in Environmental Studies and Biology. Post graduation, she plans to spend the summer building community with middle school children in Vietnam through the Coach for College program. Next fall, she will be attending Umass Amherst’s School of Public Health and Health Sciences for a masters in Epidemiology. There, she hopes to use her interdisciplinary background to explore how strengthening local communities can play a role in disease control.