ANTI-OXIDATIVE DEFENSE IN A MARINE POLAR CILIATE, EUPLOTES NOBILII

Life in polar waters is permanently exposed to harsh environmental conditions, in primis sub-zero temperature, high oxygen concentration and enhanced UV radiation. These conditions are a general cause of intracellular accumulation of reactive oxygen species that are particularly deleterious to biological macromolecules. They have thus forced polar organisms to evolve specific mechanisms to cope with oxidative damage. We studied antioxidant mechanisms in a marine protist ciliate, Euplotes nobilii, in comparison with an its closely related species, E. raikovi, living in temperate seawaters. Doses and times of exposure to UV and hydrogen peroxide that did not affect at all E. nobilii viability were, instead, lethal to E. raikovi. To shed light on which genetic mechanism E. nobilii relies to face so effectively oxidative stress, particular attention was focused on methionine sulfoxide reductase (Msr) genes which are deputed to the repairing of proteins carrying oxidized methionines and basic genome components of every aerobic organism (from bacteria to mammals). Five different genes encoding type-B Msrs (specific for the reduction of methionine-sulfoxide R-forms) and five genes encoding type-A Msrs (specific for the reduction of methionine-sulfoxide S-forms) were identified from total DNA preparation of E. nobilii. The former gene set revealed unmistakable endogenous origins, since each gene carried telomeric C4A4 nucleotide repeats distinctive of every Euplotes gene of the transcriptionally active macronuclear genome. The latter gene set, instead, lacked these repeats and their sequences showed unequivocal prokaryotic origins. This observation suggested that E. nobilii reduced methionine-sulfoxide S-forms with exogenous MsrA enzymes produced by the Francisella-like bacterial endosymbionts that appear to be habitual host of its cytoplasm. We are currently verifying this hypothesis by screening the genome of the endosymbionts that we have successfully isolated and cloned into stable laboratory colonies.