Astrobiological aspects of the mutagenesis of cosmic radiation on bacterial spores

Based on their unique resistance to various space parameters, Bacillus endospores are one of the27 model systems used for astrobiological studies. In this study, spores of B. subtilis were used to study28 the effects of galactic cosmic radiation on spore survival and induced mutagenesis. In interplanetary29 space, outside Earths protective magnetic field, spore-containing rocks would be exposed to30 bombardment by high-energy charged particle radiation from galactic sources and from the sun,31 consisting of photons (X-rays, gamma-rays), protons, electrons and heavy, high-energy charged (HZE)32 particles. B. subtilis spores were irradiated with X-rays and accelerated heavy ions (Helium, Carbon,33 Silicon and Iron) in the linear energy transfer (LET) range of 2 to 200 keV/um. Spore survival and therate of the induced mutations to rifampicin-resistance (RifR34 ) depended on the LET of the applied35 species of ions and radiation, whereas the exposure to high-energy charged particles, e.g. iron ions, ledto a low level of spore survival and increased frequency of mutation to RifR 36 compared to low-energycharged particles and X-rays. Twenty-one RifR 37 mutant spores were isolated from X-ray and heavy ionirradiatedsamples. Nucleotide sequencing located the RifR 38 mutations in the rpoB gene encoding the beta-39 subunit of RNA polymerase. Most mutations were primarily found in Cluster I and were predicted to40 result in amino acid changes at residues Q469L, A478V, and H482P/Y. Four previously undescribed41 alleles in B. subtilis rpoB were isolated; L467P, R484P, and A488P in Cluster I and H507R in thespacer between Clusters I and II. The spectrum of RifR 42 mutations arising from spores exposed to43 components of galactic cosmic radiation is distinctly different from those of spores exposed to44 simulated space vacuum and Martian conditions