757 ISSN 0026-8933, Molecular Biology, 2020, Vol. 54, No. 5, pp. 757–768. © Pleiades Publishing, Inc., 2020. Russian Text © The Author(s), 2020, published in Molekulyarnaya Biologiya, 2020, Vol. 54, No. 5, pp. 858–871. Hyper-Resistance of the Bacillus licheniformis 24 Strain to Oxidative Stress Is Associated with Overexpression of Enzymatic Antioxidant System Genes D. S. Karpov a, *, P. G. Osipova b , A. I. Domashin b , N. B. Polyakov c, d , A. I. Solovyev c , M. V. Zubasheva c , V. G. Zhukhovitsky c, e , V. L. Karpov a , S. V. Poddubko b , and N. D. Novikova b a Center for Precision Genome Editing and Genetic Technologies for Biomedicine, Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, Moscow, 119991 Russia b State Scientific Center of the Russian Federation, Institute of Bio-Medical Problems, Russian Academy of Sciences, Moscow, 123007 Russia c Gamaleya National Research Centre for Epidemiology and Microbiology, Ministry of Health of the Russian Federation, Moscow, 123098 Russia d Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, 119334 Russia e Sechenov First Moscow State Medical University, Ministry of Health of the Russian Federation, Moscow, 119991 Russia *e-mail: aleom@yandex.ru Received April 20, 2020; revised May 15, 2020; accepted May 15, 2020 Abstract—At the International Space Station (ISS), artificial living conditions are created and maintained to satisfy human needs, these conditions are also favorable for the growth of numerous microorganisms, molds and bacteria. Among the microorganisms detected on the ISS are those from the automicroflora of crew members, and a significant number of spore-forming bacteria. In most cases, this group of microorganisms gives rise to strains that are able to colonize, grow and reproduce on interior materials and equipment of sta- tions, and may be involved in biodestructive processes. These bacteria show increased resistance to various stress factors, for example, DNA-damaging and oxidizing agents. The molecular mechanisms of this resis- tance to stress are poorly understood. As part of the sanitary-microbiological monitoring of the ISS habitat, the Bacillus licheniformis 24 strain was isolated. Here, we demonstrated that this strain has increased resis- tance to hydrogen peroxide and Paraquat when compared to the “terrestrial” B. licheniformis B-10956 strain. B. licheniformis 24 overexpressed genes encoding enzymes that neutralize reactive oxygen species, such as KatX catalase and the superoxide dismutases SodA and SodF . Apart from this, in comparison with B. licheni- formis B-10956, of B. licheniformis 24 cells had lower hydrogen sulfide production that was associated with sharply reduced expression of the cysIJ operon that encodes sulfite reductase. The results indicate that enzy- matic antioxidant protective systems make a more significant contribution to the hyper-resistance of Bacillus strains to oxidizing agents than components of non-enzymatic systems, such as hydrogen sulfide. Keywords: Bacillus licheniformis 24, international space station, oxidative stress, hydrogen sulfide DOI: 10.1134/S0026893320050040 INTRODUCTION Orbiting space stations create specific conditions for microorganisms. For example, in the orbit of the International Space Station (ISS), the level of ionizing radiation is increased, which leads to more intense damage to the genetic material of microbial cells [1]. In addition, there is a lack of nutrients [2], and sur- faces of the interior and equipment of the station are periodically treated with antimicrobial and sporicidal reagents [3]. Despite these factors, which are harmful for most types of prokaryotes, some microorganisms nevertheless populate the internal volumes of the space station [4, 5], but their species diversity is sig- nificantly reduced [6]. The species composition of the station microbiome is being studied for sanitary- microbiological monitoring of the state of the living environment of the ISS crew [5‒8]. It is expected that the bacterial strains found inside the ISS have increased resistance to various stress factors compared to “terrestrial” strains. For example, we previously described the Bacillus subtilis 20 strain, which is hyper-resistant to the action of DNA damaging factors and some oxidizing agents [9]. In addition to the ISS, representatives of the Bacillus genus, which are hyper- resistant to ultraviolet radiation and oxidative stress, are also found in regularly sterilized rooms where spacecraft are assembled [2]. The B. pumilus SAFR-032 strain is particularly resistant to oxidative stress [3]. At present, MOLECULAR CELL BIOLOGY UDC 577.21:579.23