Rubrobacter radiotolerans

Classification

Higher order taxa

Domain: Bacteria

Phylum: Actinobacteria

Class: Actinobacteria

Subclass: Rubrobacteridae

Order: Rubrobacterales

Family: Rubrobacteraceae

Genus: Rubrobacter

Species

Rubrobacter radiotolerans RSPS-4 (Type Strain RRD 42256)

Description and significance

The strain RSPS-4 of Rubrobacter radiotolerans was isolated from hot spring runoff in São Pedro do Sul, Portugal (Egas, et al., 2014). The spring was at the surface and had a pH of 8.9 and a temperature of 50℃. From this and isolation it seems to have optimal conditions in semi-salty water with temperatures of 45℃ (hot springs). The strain was warranted to the the Spanish Type Culture Collection (CETC) under the code CETC 8386 (Egas, et al., 2014). Rubrobacter radiotolerans (at first, Arthrobacter radiotolerans) was originally found in a water sample taken from a hot spring in Tottori Prefecture, Japan. It was discovered after putting the sample through gamma-irradiation (Strain P-1) (Yoshinaka, Yano, and Yamaguchi, 1973). It is a gram-positive bacterium high in G+C content that is non-sporulating and non-motile. In earlier stages the bacteria seems to be shaped mostly as pleomorphic rods and in late stages seems to be mostly coccoid-shaped or shaped as short rods (Yoshinaka, Yano, and Yamaguchi, 1973). R. radiotolerans are smooth, opaque, and contain pinkish to red pigmented carotenoids. The major pigmented components are bacterioruberin and monoanhydrobacterioruberin which have 13 conjugated double bonds that are characteristic of halophilic bacteria. Bacterioruberin has four tertiary OH groups and monoanhydrobacterioruberin has three tertiary OH groups (Saito et al., 1994). A third major component, deinoxanthin, has also been identified in contributing to their reddish pigment (Asgarani et al., 2000).

R.radiotolerans are extremely resistant to UV, thermal, and gamma radiation, even more so than its relative Rubrobacter xylanophilus (Ferreira et al., 1999). Gamma radiation is typically restricted to locations contaminated with nuclear waste as high doses are not naturally found in the biosphere. One hypothesis for such a characteristic is that it evolved in response to environmental challenges such as desiccation or reactive oxidative stress. In addition, such radiation resistance is often found in organisms that do not produce spores, such as R.radiotolerans (Egas et al., 2014).

The ability to recover from such high doses of radiation seems to result from a combination of several mechanisms. Within its carotenoids, the components deinoxanthin and bacterioruberin are attributed to radiation resistance and act as an effective antioxidant in vitro. Additionally, bacterioruberin has also been attributed to resistance to hydrogen peroxide. A DNA repair enzyme, known as R-endonuclease, aids in DNA repair activity by recognizing radiation-induced DNA lesions. Thymine glycol and ring fragmentation products such as urea residues, which blocks DNA replication and is premutagenic, and abasic sites, which can be lethal and mutagenic, are cleaved by this repair enzyme in order to prevent their cytotoxic and mutagenic effects on cells (Asgarani et al., 2000). A third mechanism involves the manganese superoxide dismutase (SOD), an enzyme that forms homo-tetramerization of 24,000 Da-monomer while maintaining enzymatic activity against potassium cyanide and hydrogen peroxide.

Author

Amber Parcel Katie McCullough