Design and Implementation on the P2P VOD System Based on the RF-IPS Algorithm

This paper presents a P2P VOD system based on RF-IPS algorithm, and this system consists of two parts, server-side and clients. We put emphasis on the piece selection module of server-side, and in this part we cite the RF-IPS algorithm proposed in the past. In the piece selection module, we also proposed a weighting function except the RF-IPS algorithm. Using this function we will find the best server peers effectively and quickly, it will enhance the performance of this system. We carry on experiments on RF-IPS algorithm, and the results show this algorithm is efficient. Overall, we believe that the RF-IPS algorithm is feasible to further improve system performance and provide a cost-effective P2P VOD service with acceptable user experience.

Structural Analyses of Short-Chain Prenyltransfemses Identify an Evolutionarily Conserved GFPPS Clade in Brassicaceae Plants

Terpenoids are the largest and most diverse class of plant-specialized metabolites, which function in diverse physiological processes during plant development. In the biosynthesis of plant terpenoids, short-chain prenyltransferases （SC-PTs）, together with terpene synthases （TPSs）, play critical roles in determining terpenoid diversity. SC-PTs biosynthesize prenyl pyrophosphates with different chain lengths, and these compounds are the direct precursors of terpenoids. Arabidopsis thaliana possesses a subgroup of SC-PTs whose functions are not clearly known. In this study, we focus on 10 geranylgeranyl pyro- phosphate synthase-like [GGPPSL] proteins, which are commonly thought to produce GGPP [C20]. We found that a subset of members of the Arabidopsis GGPPSL gene family have undergone neo- functionalization： GGPPSL6, 7, 9, and 10 mainly have geranylfarnesyl pyrophosphate synthase activity （C25; renamed AtGFPPS1, 2, 3, and 4）, and GGPPSL8 produces even longer chain prenyl pyrophosphate （〉C30; renamed polyprenyl pyrophosphate synthase 2, AtPPPS2）. By solving the crystal structures of AtGFPPS2, AtPPPS2, and AtGGPPS11, we reveal the product chain-length determination mechanism of SC-PTs and interpret it as a ＂three floors＂ model. Using this model, we identified a novel GFPPS clade distributed in Brassicaceae plants and found that the GFPPS gene typically occurs in tandem with a gene encoding a TPS, forming a GFPPS-TPS gene cluster.

Structural Insights into the CatalyticMechanism of a Plant Diterpene Glycosyltransferase SrUGT76G1

Diterpene glycosyltransferase UGT76G1 from Stevia rebaudiana (SrUGT76G1) is key to the generation ofeconomically important steviol glycosides (SGs), a group of natural sweeteners with high-intensity sweetness. SrUGT76G1 accommodates a wide range of steviol-derived substrates and many other small molecules. We report here the crystal structures of SrUGT76G1 in complex with multiple ligands to answer howthis enzyme recognizes diterpenoid aglycones and catalyzes the 1,3-sugar chain branching. A spaciouspocket for sugar-acceptor binding was observed from the determined SrUGT76G1 structures, which canexplain its broad substrate spectrum. Residues Gly87 and Leu204 lining the pocket play key roles in switching between diterpenoid and flavonoid glucosylation. An engineered mutant of SrUGT76G1, T284S, couldcatalyze a selectively increased production of next-generation sweetener rebaudioside M, with diminishedside product of rebaudioside I. Taken together, these resutls provide significant insights into molecularbasis of the substrate specificity of scarcely documented diterpenoid glycosyltransferases and wouldfacilitate the structure-guided glycoengineering to produce diversified diterpenoids with new activities.

Molecular Basis for Sesterterpene Diversity Produced by Plant Terpene Synthases

Class I terpene synthase(TPS)generates bioactive terpenoids with diverse backbones.Sesterterpene synthase(sester-TPS,C25),a branch of class I TPSs,was recently identified in Brassicaceae.However,the catalytic mechanisms of sester-TPSs are not fully understood.Here,we first identified three nonclustered functional sester-TPSs(AtTPS06,AtTPS22,and AtTPS29)in Arabidopsis thaliana.AtTPS06 utilizes a type-B cyclization mechanism,whereas most other sester-TPSs produce various sesterterpene backbones via a type-A cyclization mechanism.We then determined the crystal structure of the AtTPS18–FSPP complex to explore the cyclization mechanism of plant sester-TPSs.We used structural comparisons and site-directed mutagenesis to further elucidate the mechanism:(1)mainly due to the outward shift of helix G,plant sester-TPSs have a larger catalytic pocket than do mono-,sesqui-,and di-TPSs to accommodate GFPP;(2)type-A sester-TPSs have more aromatic residues(five or six)in their catalytic pocket than classic TPSs(two or three),which also determines whether the type-A or type-B cyclization mechanism is active;and(3)the other residues responsible for product fidelity are determined by interconversion of AtTPS18 and its close homologs.Altogether,this study improves our understanding of the catalytic mechanism of plant sester-TPS,which ultimately enables the rational engineering of sesterterpenoids for future applications.

Occurrence and biosynthesis of plant sesterterpenes (C25), a new addition to terpene diversity

Terpenes,the largest group of plant-specialized metabolites,have received considerable attention for their highly diverse biological activities.Monoterpenes(C10),sesquiterpenes(C15),diterpenes(C20),and triterpenes(C30)have been extensively investigated at both the biochemical and molecular levels over the past two decades.Sesterterpenes(C25),an understudied terpenoid group,were recently described by plant scientists at the molecular level.This review summarizes the plant species that produce sesterterpenes and describes recent developments in the field of sesterterpene biosynthesis,placing a special focus on the catalytic mechanism and evolution of geranylfarnesyl diphosphate synthase and sesterterpene synthase.Finally,we propose several questions to be addressed in future studies,which may help to elucidate sesterterpene metabolism in plants.

Diterpenoid UDP-Glycosyltransferases from Chinese Sweet Tea and Ashitaba Complete the Biosynthesis of Rubusoside

Dear Editor,Diterpenoids are important molecules that play significant roles in many biological processes. In plants, acyclic diterpene phytol is a representative side chain of chlorophyll, and tetracyclic diterpene gibberellins ubiquitously serve as plant hormones regulating growth and development. Decoration of diterpene core struc-tures by oxidation and glycosylation are usually key steps to confer final bioactivity of diterpenoid compounds. In contrast to the extensively studied oxidases involved in gibberellin biosyn- thesis, diterpenoid-related glycosyltransferases have been scarcely documented, possibly due to the relative rarity of diter- pene glycosides in the plant kingdom. Only a few UDP-dependent glycosyltransferases （UGTs） have been characterized to participate in diterpene glycoside biosynthesis （Richman et al., 2005; Nagatoshi et al., 2012）.