Hirofumi Hashimoto

To investigate microbial viability and DNA damage, dried cell pellets of the radiation-resistant bacterium Deinococcus radiodurans were exposed to various space environmental conditions at the Exposure Facility of the International Space Station (ISS) as part of the Tanpopo mission. Mutation analysis was done by sequencing the rpoB gene encoding RNA polymerase β-subunit of the rifampicin-resistant mutants. Samples included bacteria exposed to the space environment with and without exposure to UV radiation as well as control samples held in the ISS cabin and at ground. The mutation sites of the rpoB gene obtained from the space-exposed and ISS/ground control samples were similar to the rpoB mutation sites previously reported in D. radiodurans. Most mutations were found at or near the rifampicin binding site in the RNA polymerase β-subunit. Mutation sites found in UV-exposed samples were mostly shared with non-exposed and ISS/ground control samples. These results suggest that most mutations found in our experiments were induced during procedures that were applied across all treatments: preparation, transfer from our laboratory to the ISS, return from the ISS, and storage before analysis. Some mutations may be enhanced by specific factors in the space experiments, but the mutations were also found in the spontaneous and control samples. Our experiment suggests that the dried cells of the microorganism D. radiodurans can travel without space-specific deterioration that may induce excess mutations relative to travel at Earth's surface. However, upon arrival at a recipient location, they must still be able to survive and repair the general damage induced during travel.

A wide variety of organic compounds have been found in space, and their relevance to the origin of life is discussed. Interplanetary dust particles (IDPs) are most promising carriers of extraterrestrial organic compounds, but presence of bioorganic compounds are controversial since they are so small and were collected in the terrestrial biosphere. In addition, IDPs are directly exposed to cosmic and solar radiation. Thus, it is important to evaluate the stability of organics in IDPs in space environment. We are planning a novel astrobiology mission named Tanpopo by utilizing the Exposed Facility of Japan Experimental Module (JEM/EF) of the International Space Station (ISS). Two types of experiments will be done: Capture experiments and exposure experiments. In the exposure experiments, organics and microbes will be exposed to the space environments to examine possible alteration of organic compounds and survivability of microbes. Selected targets for the exposure experiments of organic compounds are as follows: Amino acids (glycine and isovaline), their possible precursors (hydantoin and 5-ethyl-5-methyl hydantoin) and complex precursors "CAW" synthesized from a mixture of carbon monoxide, ammonia and water by proton irradiation. In addition to them, powder of the Murchison meteorite will be exposed to examine possible alteration of meteoritic organics in space. We will show the results of preparatory experiments on ground by using a UV lamp, a ⁶⁰Co source, synchrotron facilities, and a heavy ion irradiation facility.

Gravity resistance is a principal graviresponse in plants. In resistance to hypergravity, the gravity signal may be perceived by the mechanoreceptors located on the plasma membrane, and then transformed and transduced via the structural continuum or physiological continuity of cortical microtubules-plasma membrane-cell wall, leading to an increase in the cell wall rigidity as the final response. The Resist Tubule experiment, which will be conducted in the Kibo Module on the International Space Station, aims to confirm that this hypothesis is applicable to resistance to 1 G gravity. There are two major objectives in the Resist Tubule experiment. One is to quantify the contributions of cortical microtubules to gravity resistance using Arabidopsis tubulin mutants with different degrees of defects. Another objective is to analyze the modifications to dynamics of cortical microtubules and membrane rafts under microgravity conditions on-site by observing green fluorescent protein (GFP)-expressing Arabidopsis lines with the fluorescence microscope in the Kibo. We have selected suitable mutants, developed necessary hardware, and fixed operation procedure for the experiment.

JAXAが検討している火星探査において，生命探査を行う意義について述べ，具体的方法を提案した。

The Twenty-fifth Space Utilization Symposium (January 14-15, 2009: ISAS/JAXA Sagamihara, Japan)One of major problems in astrobiology is intense radiation, theoretical and experimental studies on biological protection and tolerance to radiation are needed. Air dried-microbial cells were irradiated with high-dose X-ray for 0 to 66,500 Gy and high-energy Fe ions for 0 to 2,000 Gy to examine their survivability based on the biological "energy currency", i.e., adenosine 5'-triphosphate(ATP), and the Most probably number (MPN). The well known radiation-resistant bacteria such as genus Deinococcus and Rubrobacter radiotolerans, highly stress-resistant spore former family Bacillaceae and radiation-sensitive Escherichia coli strains were used for this experiment. Irradiation of synchrotron X-ray (0.2 nm) at a high dose rate of 4.4 Gy/s (500 R/s) was done at KEK Photon Factory. All species showed exponential decrease in MPN against total dose. However, celluar ATP showed little decrease at low dose and linear (or exponential) decrease at high dose. Genus Deinococcus and Rubrobacter radiotolerans showing survival at 67 kGy, was more than 60 times resistant compared with normal E. coli. Irradiation of 500 MeV Fe ion at a dose rate of 0.172 Gy/s (2.5 x 10(exp 8) particles/s) was done at NIRS HIMAC. Generally similar results to X-ray irradiation were observed, but some genus Deinococcus and Rubrobacter radiotolerans showed no decrease at total dose 2000 Gy. And celluar ATP showed no decrease against to the dose. More biomacromolecule should be used in future study.