| Quorum sensing is a process in which,mediated by the self-produced signal(s),the microorganism can systematically regulate its popular activity.Previously,many research demonstrated that quorum sensing signals possess a huge variety in terms of molecular feature and regulatory mechanism on bacterial manner.For example,Vibrio fischeri utilizes acyl-homoserine lactone to regulate its bio luminescence process;Streptococcus pneumoniae controls the development of competent state by a peptide signal;while for Xanthomonas spp.,the fatty acid signals are crucial for the successful infection of their host.Xanthomonas is a phytopathogenic genus which can cause disease in?400 plant hosts including many economical crops,such as rice,citrus,cabbage,tomato and pepper.It is well studied for the role of rpf/DSF quorum sensing system played in the host-infection process of Xanthomonas.In the early stage of infection,owing to the poor environment,Xanthomonas spp.produces DSF signals at a low level and lives in a sessile state;along with the development of disease symptom,DSF signals'production increases with the support of leaked nutrition from plant cells and Xanthomonas spp.switches from sessile multicellular to motile single-cellular lifestyle.The present researches showed that Xanthomonas spp.can produce three DSF family signals with distinct fatty acid structure and their production proportion was determined by the composition of media.Recently,RpfF had been characterized as the key enzyme to dissociate BDSF from the intermedia of fatty acid biosynthesis in Burkholderia cenocepacia.However,the work relating synthesis of DSF family signals with fatty acid synthesis cycle is limited,and the precursor(s)for biosynthesis of DSF family signals is still not determined.This study investigated the biosynthesis of DSF family signals in Xanthomonas campestris pv.campestris and the results obtained are listed below.1)Four FabG domain containing proteins are found through systemically analysis of SDR superfamily proteins from Xanthomonas spp..Three of them(namely Xcc1018,Xcc4003 and Xcc0384)present widely in Xanthomonas,while Xcc0416 is limited to Xcc and its relative strains.With bioinformatics prediction of the operon where the individual gene may exist,Xcc0384 was found to share the same operon with Xcc0383,which encodes the BioC protein for biotin synthesis.And the promoter region of Xcc0416 was predicted with three binding motifs of Clp,the global transcription factor in Xanthomonas.2)Through genetic analysis(gene deletion and over-expression)of the four potential fabG respectively,we found that they all are functionally related with the biosynthesis of DSF family signals and they can affect the production of signals in distinct manners.3)Testing the effect of synthetic media on the production of DSF family signals,we identified isoleucine as the precursor for the biosynthesis of a new DSF family member(named IDSF),which was then purified and characterized as cis-10-methyl-2-dodecenoic acid by means of HPLC and NMR analysis.In conclusion,our study presents more details for the biosynthetic pathway of DSF family signals and in Xanthomonas campestris pv.campestris.The living environment of bacteria is usually changeable and,to adapt to the unstable surroungdings,bacteria has evolved distinct sensors termed two-component system to deploy nutrient-harvesting factors,such as transport systems and enzymes,and adjust its metabolism,and then balance its growth rate.The two-component system PhoB/R is used by bacteria to cope with phosphate starvation and the signaling mechanism of PhoB/R in Eschrichia coli was well characterized,however little is known for the role(s)of PhoB/R played in growth rate and pathogenicity control of Xanthomonas,one of the phytopathogenic genus which can infect?400 plants including many economic crops.With the help of transcriptome and metabolome tools,this study is set to reveal the regulatory mechanism of PhoB/R on the growth rate and pathogenicity of Xanthomonas oryzae pv.oryzae PX099A,which was observed by previous work.We noticed that the content of phospholipid in phoR mutant is significant lower than that of it in wild-type strain;what is intriguing is that the phospholipid content in phoB mutant is slight higher than that in wild-type strain.However there is no difference for the content of non-phosporus lipids between phoB/R mutants and wild-type strain,although they only take a minor part in the total membrane lipid detected.Supporting with transcriptome and metabolome data,we hypothesize that changes of membrane phospholipid is,if not the only,the main cause of comprimised growth rate and pathogenicity of Xoo.This study indicate that,after receiving the signal from pst system(needed to be proved in Xoo latter),PhoB/R is going to remodel membrane lipid of Xoo,and it will also regulate its transport systems and carbonhydrate metabolism as revealed by transctiponme analysis. |
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