| A marine cyanobacterium Synechococcus sp．PCC 7002 was exploited as majororganism object to investigate the physiological roles mad the biosynthesis pathway ofcarotenoid．Targeted mutations ofthe enzyme genes for the carotenoid biosynthesis gave a setof xamthophyll mutants,which formed the carotenoid pigment composition with either acertain type of xanthophyll with B—carotene or totally xanthophyll—free．Xanthophyll mutantsshowed growth inhibition under low light intensity．When cultured under the light intensitieshigher than standard condition,the lack of xamthophyll carotenoids generated varied degreesof growth defect,decreased cellular pigment content and photosystem I(PS I),andincreased levels of reactive oxygen/nitrogen species(ROS/RNS)．The mutant without anyxanthophyll under super—saturating light even could accumulate as higher as about 900 timesROS/RNS level relative to that of wild type(WT)under normal light．Multi—locus mutationsof the xanthophyll biosynthesis genes caused cumulative effects on the accumulation ofROS/RNS,decreased pigment content,reduced growth rate and decreased fitness of theorganism．All of the six major xamthophylls in Synechococcus 7002 had indispensible roles in theresponse and even the maintenance of cellular viability when under photo—oxidative stress．When compared from a lateral view,the missing of hydroxyl—and keto—carotenoids such aszeaxanthin and echinenone led to a higher degree of decreased chlorophyll content and elevatedROS/RNSlevelthanthosefromthe oxidative stress duetothe removals ofothertwo xaxlthophyllpathways,this observationmatchedthe assumptionthatthetwohydroxyl—andketo—xyanthophyllswere located on photosynthetic apparatus and may directly involve in the protection ofthylaJ~oid against oxidative stress．Therefore,it may suggest that those hydroxyl—andketo—carotenoids,catalyzed by hydroxylase CrtR and ketolase CrtW respectively,playedgreater roles in response to intrinsic oxidative stress and damage in the actively growing cells．Whereas the newly discovered carotenoid diacid synthechoxanthin and glycosylatedmyxoxaxlthophyll,produced with the p—carotene desamrase/methytransferase CruE mad1'hydroxylase CruF,had a more important role in the protection ofextracellular stress．Synechococcus 7002 provides a mature expression platform,which could be combinedwith the above xamthopyhll mutants as genectic backgrouds to expand its carotenoid pathway．A carotenoid ketolase gene crtW from a newly identified thermophilic anaerobic bacteriaIgnavibacterium album was overexpressed in the Synechococcus 7002 crtW mutant and showed in vivo enzymatic activity,which conve~ed p—carotene to echinenone and3'-hydroxyechinenone and complemented the intrinsic deactivated ketolase of Synechococcus7002．However,the expressions of the i album crtW as well as another two cyazmbacteriumcrtW genes from Anabaena PCC 7120 and Nostoc punctforme PCC 73102 had not yet beenobservedtoproduce astaxa=thinoritsintermediates otherthanthose pigmentsfromthebackgroundstrains．The lycopene cyclases CmA/CmP in Synechococcus 7002 discovered in recent year aledifferentfrom anyknowntypes ofcarotenoid cyclase．Overexpressions ofCmAinwi dtype,cruAmutant and cruP mutant displayed its/n vivo activity of catalyzing lycopene and 7-caJcotene intop—carotene．The CruA was purified by affinity chromatography from different cellularfractions and chlorophyll may serve as its cofactor,this experimental result together with thein vitro assay may help disclose the catalysis mechanism for this new type of lycopenecyclases．Synechococcus 7002 possesses the expandable capacity to investigate its owncarotenoid biosynthesis pathway and the applications such as producing new or specificalvalued xanthophylls．...
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