Importance of catalase in adaptive response and resistance to hydrogen peroxide in Haloferax IRU1

摘要

Haloferax IRU1 is an extremely halophilic Archaeon, which is highly resistant to the lethal effects of hydrogen peroxide. To investigate the role of catalase in the tolerance of Haloferax IRU1 to hydrogen peroxide stress, theexpression and activity of catalase by this archaeon was studied. When cells were treated with several levels of hydrogen peroxide, activity and expression of catalase increased, in comparison to that of untreated cells. This suggests that catalase is an important factor in the tolerance of Haloferax IRU1 to hydrogen peroxide stress.To investigate the adaptive response to the lethal effects of hydrogen peroxide in Haloferax IRU1, cultures when pretreated with sub-lethal levels of hydrogen peroxide, became more resistant to its lethal effects and exhibited higher levels of catalase than those of un-pretreated cultures. This indicates that catalase is induced during the adaptive response of this strain to hydrogen peroxide stress. Introduction Woese and Fox have proposed that all organisms can be classified into three domains: Archaea, Bacteria and Eucarya (Woese and Fox1994). Members of the domain Archaea are phylogenetically divided into three kingdoms, namely, Euryarchaeotoa, Crenarchaeota and Korarchaeota. Most members of the kingdom Euryarchaea are predominantly methanogens, but there are two other phenotypes found in this group, sulfur metabolizing thermophiles and extreme halophiles (Danson et. al. 1992).The extremely halophilic Archaea require at least 2M NaCl or equivalent ionic strength for growth and the most growth is in saturated or near-saturated brines (Madigan et. al. 2000). Haloferax IRU1 is an extreme halophilic member of the kingdom Euryarchaeotoa, that was obtained by filtration of the Uromia salt lake water located in the north-west of Iran. After microscopic, morphological and biochemical studies and determination of the optimal concentration of salts required for growth, this microorganism was identified as Haloferax IRU1. It shows high resistance to ionizing radiation (gamma rays), non – ionizing radiation (Ultraviolet light) and to the oxidative agent hydrogen peroxide (H2O2) (Asgarani et. al. 2007). Ionizing radiation and many other oxidative agents such as hydrogen peroxide produce different types of reactive oxygen species (ROS) including the superoxide anion (O.-2), hydrogen peroxide (H2O2) and hydroxyl radical (.OH) will be generated (Halliwell 1994). These species are capable of damaging DNA, protein and lipid membranes, sometimes causing lethal damage to cells (Cadenas 1989). They can be produced by both environmental and endogenous sources (Halliwell 1994).Considering the high resistance of Haloferax IRU1 against oxidative stress, it is reasonable to assume the presence of developed defensive mechanisms in this organism that allow cells to survive after exposure to high levels of ionizing radiation and hydrogen peroxide. Enzymatic defenses include superoxide dismutase, catalase and peroxidases (Gaetani et. al. 1989). Superoxide dismutase, converts superoxide radicals to H2O2, and catalase converts H2O2 to water and oxygen (Brioukhanov and Netrusor 2004). Under normal physiological conditions there is a balance between ROS generation and their elimination by different antioxidants. If the production of oxidants is enhanced or the power of the antioxidant system is decreased, oxidative stress arises (Fridowich 1978). Cells pre-exposed to comparatively mild and sub lethal stress situations activate adaptive responses and acquire tolerance to subsequent more lethal stress. Such responses, named adaptation, are observed not only in bacterial cells but also in eukaryotic organisms (Crawfold and Davics 1994). Detailed mechanisms of adaptation to oxidative stress have been most extensively characterized in bacterial cells (Engelmann and Hecker 1996) and have also been studied in yeast (Mutoh et. al. 1995, Mutoh et. al. 1999).An advantage of using an archaeon as a model system for stu