Studies On The Function Of Skeleton In Calcimycin Biosynthesis Related Gene And High-efficiency Expression Of Glucose Isomerase

Calcimycin(A23187)is a pyrrole polyether antibiotic discovered from Streptomyces chartreusis NRRL 3882 in 1972.Calcimycin has not only wide-range antimicrobial activity(effective inhibition of Gram-positive bacteria and yeast),but also the activities of ATPase inhibition and apoptosis induction of cultured mammalian cells.Recent research results showed that Calcimycin also efficiently inhibits metastasis formation in xenografted immunodeficient mice.The diversity of Calcimycin activities is determined by its unique structure.Calcimycin is composed of three typical parts:a pyrrole ring,a spiroketal ring and a substituted benzoxazole Calcimycin biosynthesis gene cluster was reported in 2010,wherein,only genes involved in biosynthesis of the pyrrole ring were identified,the genes involved in biosynthesis of spiroketal ring and benzoxazole ring are still unknown.This study investigates the mechanism of biosynthesis of spiroketal ring and benzoxazole ring,and further elucidates the whole Calcimycin biosynthetic pathway.The function elucidation of calcimycin biosynthetic related enzymes can be used to mine new natural products by using combinatorial biosynthesisThere are five type-I PKS genes,calA1?calA2,calA3,calA4 and calA5 in Calcimycin biosynthesis gene cluster.The four PKS genes of calA1?calA2?calA4 and calA5 are responsible for the extension of the polyketide chain.Analysis of fermentation results of ?calA3 mutant showed that Calcimycin was not detected in?calA3 mutant,but detected the accumulation of Cal-3.Comparative analysis of Calcimycin,Cezomycin and Cal-3 showed that Cal-3 lacks 3-hydroxyanthranilic acid(3HA)of Calcimycin in ?calA3 mutant.This implies that CalA3 is involved in the attachment of the 3-hydroxyanthranilic acid(3HA)units,which further form Calcimycin by subsequent catalysisTo figure out the function of CalA3,SNAC-Cal-3 was used in the enzymatic reaction catalyzed by CalA3 protein with 3HA.But there in no product detected in above in vitro reaction with substrates SNAC-Cal-3 and 3HA.Polyketide analogues Cal-7 was bought,and further synthesized SNAC derivative Cal-8.Similarly,in the vitro experiment using Cal-8 and 3HA as substrates,we found that CalA3 catalyzes condensation reaction between Cal-8 and 3HA to form Cal-11,meanwhile,CalA3 can also hydrolyzes Cal-8 to produce Cal-7.Comparative amino acid sequence analysis of CalA3 showed that Cal A3 contain four domain,KS,AT,DH and KR domain Compared to active sites of known function domains,the active sites of AT,DH and KR domains of Ca13 are not conserved to those domains.We speculated that the AT,DH and KR domains might not participate in theses catalytic reactions.Experiment of Amino acid site-specific mutagenesis(C164A,H299A and H339A)in KS of CalA3 showed that the mutants of CalA3 have no activity to catalyze the hydrolysis and C-N bond formation.Those results implied that CHH catalytic triad of KS in CalA3 is responsible for the hydrolysis and C-N bond formation.At the same time,these active sites of CHH catalytic triad in KS domain of CalA3 are consistent with published canonical type I PKS active sitesCalA3 has broad substrate selectivity in vitro.It efficiently catalyzes the reaction with the SNAC derivative of either pyrrole ring or thiazole ring in the substrate of polyketide chain.CalA3 recognizes different amino donors and hydroxyl donors,such as 3HA,glycerol or methanol,et al,to form products which contain C-N bond or C-O bond.Our studies proved that CalA3 is a PKS enzyme with novel catalytic activity and broad substrate selectivity,which can be used in combinatorial biosynthesis research and mining new natural productsTo understand the biosynthesis of spiroketal ring in Calcimycin,?calU2 mutant strain was fermented and fermentation products were isolated and purified.Analysis of the fermented products showed that ?calU2 mutant strain accumulated two new compounds:Cal-4 and Cal-5.In order to resolve their structures,we tried to separate Cal-4 and Cal-5 from ?calU2 fermentation liquid.It turned out that both Cal-4 and Cal-5 are unstable,and were unable to purify enough for NMR identification analysis.Cal-4 transforms to Cal-3 spontaneously,and Cal-5 transforms to Cal-6 spontaneously.Based on LC-MS analysis,we speculated that both Cal-4 and Cal-5 are compounds before the formation of the spiroketal ring.In vitro experiment with purified CalU2 protein showed that CalU2 catalyzes the enzymatic reactions of Cal-4 and Cal-5 to form Cal-3 and Cal-6,respectively.Those results showed that CalU2 is the key enzyme responsible for the spiroketal ring formation in Calcimycin biosynthis pathwayThe formation of C-N bond is the first step of formation of benzoxazole in Calcimycin biosynthesis,and benzoxazole ring is further formed through dehydration and cyclization of benzoxazole.To identify genes involved in dehydration and cyclization,all genes in Calcimycin biosynthetic gene cluster were bioinformatically analyzed.An unknown function PKS cyclase gene(calU1)was found.Fermentation products analysis of ?calU1 mutant strains by HPLC and LC-MS analysis showed that the amount of Calcimycin decreased dramatically in?calU1 mutant strains,but accumulate a new product Cal-6 substantially,compared to those in wild-type strain.Cal-6 was purified for NMR analysis and its structure was resolved.Comparative structure analysis of Cal-6 and Cezomycin suggests that Cezomycin might be formed from Cal-6 by dehydration and cyclization.CalU1 was heterologously expressed and purified for in vitro catalytic reaction experiment Cezomycin was not detected from this reaction with compound 2 as substrate under the catalyzatoin of CalU1.This reaction of dehydration cyclization needs to be further studiedOur research further illuminated the Calcimycin biosynthesis pathway.After the Polyketide chain was biosynthesized,then catalyzed by CalA3 to form C-N bond through ligation to 3HA,further catalyzed by CalU2 to form spiroketal ring through dehydration reaction,finally is catalyzed to form benzoxazole ring,resulting in mature Calcimycin by post-modification.Our study on the new function of type I PKS promises insights into a new enzymatic reaction towards discovery and creation of novel natural products through combinatorial biosynthesis and synthetic biology approach.