The Mechanism Of Nostoc Flagelliforme Adaptation To Terrestrial Habitat

Desiccation-tolerant cyanobacteria living in arid and semi-arid desert,often dominated by a form species of Leptolyngbya,Microcoleus,Tolypothrix,Calothrix and Nostoc.They frequently experience long-term harsh conditions such as desiccation,high ultraviolet radiation,and hydration/dehydration cycles,can able to form akinetes to adapt the dynamically changing extreme environments when suffered to long desiccation and contribute significantly to form biological desert soil crusts(BSCs)for controlling desertification.However,the molecular and genetic backgrounds of desiccation-tolerant and adaptation mechanisms of harsh environment are still not well understood,which is hampered by the lack of genome sequences assembled completely.To fill this critical gap,we sequenced the genome of free-living desiccation-tolerant terrestrial cyanobacterium,Nostoc flagelliforme CCNUN1.Using genomics and transcriptomics approach,together with comparative genomics,RNA expression profiling analyses,and molecular genetic,we show that the the mechanistic aspects of desiccation tolerance and adaptations of the dehydration/rehydration cycle,as well as the regluation mechanism of MAA biosynthesis linking UV-B signaling.The main results are as follows:1.Genome sequencing and comparative genomics analysis.The genome of N flagelliforme CCNUN1 was sequenced to completion by a hybrid approach of 454 GS FLX/llumina HiSeqTM 2000/PacBio RS ?.The DNA content of N.flagelliforme CCNUN1 is composed of 10.23 Megabases(Mb),and contains 10,825 predicted protein-coding genes,makes it the largest complete genome reported to date among the cyanobacterium.Comparative genomics analysis ammong 20 cyanobacteria reveals that heavy expansion of genes related to Mobilome/DNA replication/recombination/repair,and cell cycle control/cell division/chromosome partitioning,which could account for a great evolutionary flexibility and accommodating hydration/dehydration cycles.In addition,genome mining offers some important insights into the genetic strategies of genomic adaptation to abiotic stress tolerance,includes the acquisition through horizontal gene transfer of genes involved in regulation of exopolysaccharide synthesis,adaptation to ultraviolet radiation environments and responsing to DNA repair and oxidative stress.To gauge the natural product potential of N.flagelliforme CCNUN1,we identified 60 secondary metabolite biosynthetic gene clusters.2.Gene expression profiling upon rehydration.An analysis of the dynamic transcript profiling during the rehydration,we established the temporal gene expression patterns and developed some uniqe mechanisms to adapt to rapid resuscitation upon wetting.Transient increases in expression of genes involved in cyclic electron transport around photosystem I that generates a transmembrane electrochemical gradient is used by the ATP synthase to produce ATP,which has been showed that it is necessary for the recovery and rapid re-initiation of photosynthesis immediately after rehydration and metabolism.Except that,transient increases in expression of genes involved in signal transduction/bacteriophytochrome that possibly transduce the light signals to rapidly restart its metabolism,oxidative stress that scavenge ROS and protect DNA and proteins from oxidative damage,chemotaxis that control the transient cell aggregation and swim via the chemotaxis pathway.The whole-genome expression profiling in N.flagelliforme wild populations revealed much more genes transient increase in expression involved in photosynthesis,such as PSI,PSII assembly factors(LTO1,psb32,ctpA,psb27 and CyanoP),oxygen-evoling complex(OEC)(psbO,psbU,psbV and psbP),allophycocyanin(apcA,apcB,apcD and apcF),phycocyanin(cpcA and cpcB),and phycobilisome linker proteins(cpcC,cpcG,apcC and cpcD).The transient increase in expression of genes encoding for spore germination protein GerM and transient decrease about akinete-marker genes showed that the akinete in dormant state exhibits rapid resuscitation upon wetting.The oneset of rehydration triggered the downregulation of genes involved in photo-protective such as psbA(Q130E),high light inducible protein(Hlips),Orange Carotenoid Protein(OCP),flavoproteins(Flvs),NblA,carotenoid biosynthesis and UV-protectants such as MAAs,scytonemin,motility and chemotaxis and ROS-protectants such as Mn-containing catalase and desiccation-tolarance such as WspA and nitrogen fixation such as nifH,nifD,nifK and nifE.Reversely,maybe theses genes play important role in adapting to extreme environment,often subjected to long desiccation,high irradiance.3.Transcriptional regulation of the MAAs biosynthesis genes.we identified a unique five-gene cluster for MAA biosynthesis acquired through horizontal gene transfer in desiccation-tolerant.A luxR family protein OrrA was identified as a positive UV-B responsive regulator binding to the promoter region of this gene cluster in Nostoc fagelliforme CCNUN1.OrrA functions as a UV-B activator mediating the UV-B induced five-gene cluster expression and MAA biosynthesis.Furthermore,ORF5811 was identified as a candidate upstream tansduction component of OrrA linking MAA biosynthesis assaied by pull-down and yeast two-hybrid.In conclusion,our completion of the genome sequence and transcriptome upon rehydration of N.flagelliforme,provides this genetic infrastructure to understand the mechanistic aspects of desiccation tolerance and adaptations of the dehydration/rehydration cycle and artificial culture for resource restoration.Moreover,the newly identified OrrA and ORF5811 enhances our understanding about UV-B signal transduction linking MAA biosynthesis.