De Novo Transcriptomic Analysis Of Races A And L Of Botryococcus Braunii Species

The colonial microalga Botryococcus braunii has been brought to people's attention for its conspicuous ability to accumulate a variety of lipids including hydrocarbons.B.braunii strains are classified into 3 races based on the types of hydrocarbons.However,their extreme slow growth rate has impeded its application for biofuel production.We report the transcriptomic response of a moderately growing subisolate B.braunii 779 upon nitrogen deprivation(ND).We show that the subisolate has an average growth rate of 0.52 g l-1 day-1 under photoautotrophic growth conditions and lipid content is enhanced to 75%of CDW upon ND.We found that many homologous ESTs between A races 779 and Bot-88 are unknown sequences,implying that A race contains many unknown genes.Pathway-based transcriptomic analysis indicates that energy metabolisms are among the top expressed functions in log-phase cells,indicating that the slow growth rate is a result that energy flow is directed to lipid biosynthesis but not population growth.Upon ND,reconfiguration of metabolisms for reducing power is apparent,suggesting that 779 is rapidly adapting under ND condition by transcriptomic reprogramming.We also report the de novo assembly of transcriptome in race L B.braunii AC768.We show that transcriptome of race L shares similar numbers of mutual best-hits(?20%)with that of race A and race B.Sequence alignment analysis of squalene synthase-like proteins indicates that lycopaoctaene synthase proteins catalyzing the first reaction to produce lycopadiene from pre-phytoene diphosphate are specific to race L.Additionally,botryococcene synthase proteins are unique to race B.Taken together,our result shows that A,B and L races may belong to different sub-species.Furthermore,there are many novel genes that are unique to A,B and L races respectively,suggesting that sequences of many enzymes involved in hydrocarbon biosynthesis are not currently known.We propose that B.braunii transcriptomes provide a rich source for discovery of novel genes involved in hydrocarbon biosynthesis.