Astrobiological Aspects of the Mutagenesis of Cosmic Radiation on Bacterial Spores

Abstract

Based on their unique resistance to various space parameters, Bacillus endospores are one of the model systems used for astrobiological studies. In this study, spores of B. subtilis were used to study the effects of galactic cosmic radiation (GCR) on spore survival and induced mutagenesis. In interplanetary space, outside Earth's protective magnetic field, spore-containing rocks would be exposed to bombardment by high-energy charged particle radiation from galactic sources and from the Sun, which consists of photons (X-rays, γ rays), protons, electrons, and heavy, high-energy charged (HZE) particles. B. subtilis spores were irradiated with X-rays and accelerated heavy ions (helium, carbon, silicon and iron) in the linear energy transfer (LET) range of 2–200 keV/μm. Spore survival and the rate of the induced mutations to rifampicin resistance (Rif^R) depended on the LET of the applied species of ions and radiation, whereas the exposure to high-energy charged particles, for example, iron ions, led to a low level of spore survival and increased frequency of mutation to Rif^R compared to low-energy charged particles and X-rays. Twenty-one Rif^R mutant spores were isolated from X-ray and heavy ion–irradiated samples. Nucleotide sequencing located the Rif^R mutations in the rpoB gene encoding the β-subunit of RNA polymerase. Most mutations were primarily found in Cluster I and were predicted to result in amino acid changes at residues Q469L, A478V, and H482P/Y. Four previously undescribed alleles in B. subtilis rpoB were isolated: L467P, R484P, and A488P in Cluster I and H507R in the spacer between Clusters I and II. The spectrum of Rif^R mutations arising from spores exposed to components of GCR is distinctly different from those of spores exposed to simulated space vacuum and martian conditions. Key Words: Bacillus—Spores—High-energy charged particle bombardment—Lithopanspermia. Astrobiology 10, 509–521.