The MS2 gene of Paramecium tetraurelia is supposed to be intimately implicated in the programmed association between sexual reproduction and life span and in the eventual setting of its maximum life span. The transcript of this gene is presumably an mRNA-like non-coding RNA, the function of which is evolutionarily conserved in its specific secondary structure. Therefore, characterizing the secondary structure of MS2 RNA would yield important insights about the molecular mechanism for the above cellular process that is shared among all of the organisms possessing limited longevity. In this study, it was found that P. tetraurelia has a close homolog of MS2 which undergoes the same regulation of expression as with MS2. We computationally analyzed the RNA secondary structures of MS2 and the homolog and detected four types of well-ordered structures that are both significantly more ordered and thermodynamically stable than anticipated at random. Our database searches revealed that these consensus structural features are conserved from other protozoan species to mammals. These results may suggest the functional importance of the distinct RNA structures in relation to the cellular program for organismal life span.

Endosymbiotic algae of Japanese Paramecium bursaria were isolated and their axenicity was finally confirmed by DGGE method. Japanese symbiotic strains (F36-ZK and others) and American one, NC64A from ATCC, could use ammonia and organic nitrogen but not nitrate or nitrite as a sole nitrogen source. The measurement of nitrate (NR) reductase activity of NC64A and F36-ZK revealed the difference between these symbionts, i.e., weak NR activity was detected in NC64A cell extract but not in F36-ZK. Since Japanese symbionts preferred several amino acids for their growth, amino acid uptake were studied. F36-ZK was able to incorporate all amino acids through three amino acid transport systems, which considered to be driven by proton motive force. SSU rDNA analysis indicated four genetically discrete symbiotic algal groups depending on the strains of P. bursaria, of which three phylogenetically belonged to the Chlorellaceae, but one appeared at a different lineage in the Trebouxiophyceae. Species level analyses (ITS2 and other protein-coding genes) exposed genetic dissimilarity and polyphyletic relations of the three chlorellacean symbionts, which suggests all four types were independently captured in the evolutionary history of P. bursaria. The host cell-free extract enhanced symbiotic algal carbon fixation about 3-fold, however, release of photosynthate hardly changed. On the other hand, the release of photosynthate was obviously increased with acidic condition. Thus, the host seems to regulate the photosynthesis and the release of photosynthate of the symbiont in perialgal vacuole via a specific compound and pH in the vacuole, respectively.