Molecular Characterization of the Calvin Cycle Enzyme Phosphoribulokinase in the Stramenopile Alga Vaucheria litorea and the Plastid Hosting Mollusc Elysia chlorotica

Phosphoribulokinase （PRK）, a nuclear-encoded plastid-localized enzyme unique to the photosynthetic carbon reduction （Calvin） cycle, was cloned and characterized from the stramenopile alga Vaucheria litorea. This alga is the source of plastids for the mollusc （sea slug） Elysia chlorotica which enable the animal to survive for months solely by photoautotrophic CO2 fixation. The 1633-bp V. litorea prk gene was cloned and the coding region, found to be interrupted by four introns, encodes a 405-amino acid protein. This protein contains the typical bipartite target sequence expected of nuclearencoded proteins that are directed to complex （i.e. four membrane-bound） algal plastids. De novo synthesis of PRK and enzyme activity were detected in E. chlorotica in spite of having been starved of V. litorea for several months. Unlike the algal enzyme, PRK in the sea slug did not exhibit redox regulation. Two copies of partial PRK-encoding genes were isolated from both sea slug and aposymbiotic sea slug egg DNA using PCR. Each copy contains the nucleotide region spanning exon 1 and part of exon 2 of V litorea prk, including the bipartite targeting peptide. However, the larger prk fragment also includes intron 1. The exon and intron sequences of prk in E. chlorotica and V/itorea are nearly identical. These data suggest that PRK is differentially regulated in V. litorea and E. chlorotica and at least a portion of the V. litorea nuclear PRK gene is present in sea slugs that have been starved for several months.

Gustav Senn(1875–1945): The pioneer of chloroplast movement research

Gustav Senn analyzed for the first time light- induced movement and arrangement of chloroplasts. Using many plant species he performed physiological analyses of chloroplast migration in response to external stimuli, with emphasis on light. He determined light paths within a cell by measuring refractive indices and optical thickness of cellular compartments and confirmed that chloroplasts migrate towards the region where the light intensity is optimum. After 6 to 7 years＇ concentrated study, Senn published the famous monograph ＂Die Gestalts- und Lageveranderung der Pflanzen- Chromatophoren＂ （The Changes in Shape and Position of Plant Chloroplasts） in 1908. This book has stimulated many plantphysiologists and photobiologists, because Senn not only thoroughly classified and defined various types of light-induced chloroplast migration but also already described possible interaction of different photoreceptor systems in Nlougeotia more than 50 years before the discovery of phytochrome. This book also contains still useful experimental hints and over- looked findings on the interaction between light and other factors, such as temperature, water content, and nourishment. After publishing this book, Senn retreated from the study of chloroplasts and became a researcher of the Greek philoso- pher, Theophrastus. In this review, I introduce his biographical background and then summarize some of his key research accomplishment.