Identity determinants of Escherichia coli tRNA^Tyr were examined using in vitro transcripts. The first and second anticodon nucleotides were base-specifically involved in tRNA recognition of tyrosyl-tRNA synthetase, whereas the third anticodon nucleotide was not. None of the identity determinants were found in the acceptor stem, except the discriminator base A73. With respect to the long variable arm, a stem of three base pairs in length was required for tyrosylation, while the sequence of the arm was not essential. From the results of identity conversion experiments, the discrimination modes of tRNA^Tyr from other amino acid specific tRNAs sharing some of the identity determinants of tRNA^Tyr were discussed.

地球に生命が誕生したのは今から４０億年程度も昔であると考えてられているが、その当時の地質学的記録はほとんど残っていない。しかし、火星表面には、地質学的記録を保持した地層が数多く残っており、生命が誕生した当時の環境が火星探査によって明らかにされる可能性がある。一方のタイタンは、非常に還元的な大気を持ち、宇宙線や紫外光などのエネルギーによって、さまざまな無生物的有機反応が起きている。タイタンにおける無生物的有機反応を詳細に調べれば、原始地球の大気に於ける分子進化過程に関する新しい知見を得られる可能性がある。本セッションでは、生命の起源に深い関連を持つこれらの２つの星について、最新の探査結果と研究紹介がなされた。

The symposium “Coevolution of earth and life” held in 30^th SSOEL-Japan meeting- ISOLAB’05 is reported. In the symposium, keynote lecture was provided by Prof. J. L. Kirschvink, who reported on the Paleoproterozoic snowball earth hypothesis. He explained the possibility that the climate disaster is triggered by the evolution of oxygenic photosynthesis. Prof. A. Yamagishi reported his recent experimental results of the last common ancestor of all the living organisms, which he named Commonote. His experimental results supported the hypothesis that the common ancestor was hyperthermophilic. Prof. S. Itoh reported the significance of the photosynthesis on the early evolution of life and earth. He proposed that the photosynthetic system has much more variation than recognized earlier. Prof. T. Hashimoto reported on his resent results of the analysis of 22 genes of eukaryotic species. He emphasized that the deep branching of three amitochondriate lineages, Microsporidia, Diplomonadida and Parabasalia are caused by the long-branch attraction, and is misleading. Prof. Y. Isozaki reported the mass extinction events in the Paleozoic. He explained the possible relation of mantle plume and Paleozoic-Mesozoic extinction. These reports illuminated the ever-increasing knowledge in each of earth science and life science, though there are still many unsettled controversial issues. However, every audience have noticed that these two fields of science can’t proceed ignoring the other and that the clearer picture of early history of earth and life is now emerging by the concomitant progress of the two sciences.

Since 1995, more than 160 extrasolar planets have been discovered. The recent rapid development of extrasolar planets allows us to expect existence of Earth-like planets in extrasolar planetary systems. In this session, present status and future perspective of observation of extrasolar planets and development of planet formation theory are reviewed. In particular, extrasolar terrestrial planets and possibility of extrasolar life are focused. More than 10 percent of solar type stars in our galaxy may harbor Earth-like planets having ocean on their surface. Near future space telescopes will try to detect bio-markers, such as ozone, in the planetary atmosphere.