Oxidative stress in toxicity and signalingcontrol of cysteine oxidation and reduction by the redoxin systems in fission yeast

Laburpena

Schizosaccharomyces pombe, as all aerobic organisms, needs oxygen to live and have to deal with the side effects of the toxicity associated to oxygen by-products. To maintain intracellular environment, S. pombe has antioxidant systems that are activated fast and in a reversible manner. Depending on the severity of the oxidative stress suffered, S. pombe has two main antioxidant response pathways, Pap1 and Sty1 pathways, for sub-toxic and toxic hydrogen peroxide (H2O2) levels, respectively. These cascades involve sensor proteins that contain cysteine residues called thiol switches, which can switch on and off their activities. Therefore, all the components involved in the antioxidant signaling cascades have to be recycled in order to be ready to act again. For these reason, the reduction systems, thioredoxin and glutathione/glutaredoxin systems, have an essential role as backup systems of the components of the H2O2 signaling redox relays in fission yeast. Along this thesis we have studied the importance of a proper activity of both reduction systems in the recycling of redox proteins to avoid alterations in the normal behavior of fission yeast. On the one hand, we have characterized the role of thioredoxin system as the main backup system of an essential enzyme, ribonucleotide reductase, for a normal cell cycle progression in fission yeast. We have also elucidated the role of the gene coding for glutathione reductase (Pgr1) in fission yeast, which until now was described as an essential gene. We were able to select pgr1 deletion under anaerobic conditions and characterize the deletion mutant. With regard to the study of the antioxidant systems, and the role of thiol switches in H2O2 signaling, we have deciphered the difference between an H2O2 scavenger and a sensor by comparing two model systems, one with a one-component redox relay, OxyR of Escherichia coli, versus the two-component redox relay, Tpx1-Pap1 of S. pombe, both co-expressed in fission yeast. Although it was known that reactive oxygen species (ROS) generated in cells as product of the respiratory chain, among other metabolic processes, have to be eliminated to avoid damage to biomolecules, we demonstrated that H2O2 causes direct DNA damage in fission yeast. Moreover, we characterized how important is the function of Tpx1 as scavenger in basal conditions to maintain steady state levels of ROS to avoid DNA damage in S. pombe.