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Castor Bean (Ricinus Communis L.) Genes Involved in Phytic Acid Biosynthetic Pathways: Expression Analysis in Response to Phosphate and Characterization of Inositol 1,3,4,5,6-Pentakisphosphate 2-Kinase

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Title: Castor Bean (Ricinus Communis L.) Genes Involved in Phytic Acid Biosynthetic Pathways: Expression Analysis in Response to Phosphate and Characterization of Inositol 1,3,4,5,6-Pentakisphosphate 2-Kinase
Author: Yu, Jaeju
Department: Department of Molecular and Cellular Biology
Program: Molecular and Cellular Biology
Advisor: Greenwood, John S.Bewley, J. Derek
Abstract: During seed development, myo-inositol (Ins) hexakisphosphate or phytic acid (PA) is stored in the form of phytin with mineral cations, and is mobilized following germination, releasing these nutrients that are required for seedling growth. Outside its role in seeds, PA and other phosphoylated Ins derivatives play critical roles in biological processes in many eukaryotes. PA also has negative influences on nutrition in both non-ruminant animals and humans due to its lack of digestibility. There have been two parallel PA biosynthetic pathways proposed, yet, the pathway is still poorly understood in terms of its regulation and enzymes involved. Here, genes encoding enzymes putatively implicated in castor bean PA biosynthetic pathways were identified in the genome and expression followed. Isolated castor bean embryos have the ability to resynthesize PA following germination if exogenous phosphate is available. It was found that the genes purported to act in PA synthesis were constitutively expressed in the embryos regardless of the availability of phosphate. Castor bean Ins 1,3,4,5,6-pentakisphosphate 2-kinase (RcIPK1), catalyzing the final reaction in PA biosynthesis, regardless of pathway, was chosen for further study. Even though only one copy of the RcIPK1 gene was present in the genome, numerous RNA variants were found, most likely due to alternative splicing events. These variants encoded six closely related protein isoforms based on in silico analysis. Functional analyses using yeast mutant strains lacking the IPK1 gene revealed that only three of the mRNA variants could rescue the temperature-sensitive growth phenotype, and it was demonstrated by HPLC analysis of Ins phosphates that their ability to complement the missing yeast IPK1 enzyme was associated with enzyme activity. Only these three isoforms possessed conserved motif III and motif IV important for IPK1 catalytic activity.
URI: http://hdl.handle.net/10214/5340
Date: 2013-01
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