| Inositol, usually referred to as myo-inositol, plays an important role as a precursor to many compounds in many metabolic reactions. Inositol metabolism is essential for cellular processes and development in plants, animals, algae, fungi and cyanobacteria. Derivatives of inositol are involved in signal transduction for a wide variety of hormones, growth factors, neurotransmitters and stress response to light, temperature and osmosis. Inositol metabolism is also involved in membrane formation, seed germination, cell wall biogenesis and hormone transport.; Inositol phosphate is synthesized by 1L-myo-inositol 1-phosphate synthase (EC 5.5.1.4), the only enzyme known to catalyze the conversion of glucose 6-phosphate to inositol 1-phosphate. This reaction is the rate limiting step in inositol phosphate metabolism.; The expression of 1L-myo-inositol 1-phosphate synthase has been studied in the unicellular green algae, Chlamydomonas reinhardtii , to begin to identify and isolate factors that regulate inositol phosphate biosynthesis in chloroplasts. Biochemical analyses of cells grown in culture media containing increasing concentrations of inositol suggest that 1L-myo-inositol 1phosphate synthase expression in Chlamydomonas reinhardtii is not regulated by exogenous inositol as with other organism. Inositol uptake experiments show that intact Chlamydomonas reinhardtii cells readily transport inositol phosphate, not inositol, at a rate that increases with the number of phosphates attached to the inositol ring.; Radioactive-labeled phytic acid, inositol hexakisphosphate uptake experiments demonstrate that Chlamydomonas reinhardtii cells can import intact phytic acid and utilize phytic acid as a phosphate source. Subsequent cell fraction studies show that most of the radioactive-labeled phytic acid goes into insoluble components of Chlamydomonas reinhardtii. Mutagenesis was performed to isolate mutants defective in phytic acid utilization.; A Chlamydomonas reinhardtii genomic cosmid library was constructed and a genomic clone encoding 1L myo-inositol 1-phosphate synthase was isolated. Subcellular fractionations, enzyme assays and Western blotting experiments showed that 1L myo-inositol 1-phosphate synthase expression and activity are associated with thylakoid membranes of the Chlamydomonas reinhardtii chloroplast.; These studies provide foundation needed to determine the role of 1L myo-inositol 1-phosphate synthase in chloroplasts. |
