Acetyl-CoA carboxylase(ACCase)catalyzes the biotin-dependent carboxylation of acetyl-CoA to produce malonyl-CoA,which is a key and rate-limited step in the biosynthesis of fatty acids.In order to investigate its effect on the arachidonic acid(ArA)in Myrmecia incisa grown under nitrogen starvation,two genes coding for biotin carboxyl carrier protein(BCCP)which was one subunit of the heteromeric ACCase complex,were cloned by using Rapid Amplification Complementary DNA ends(RACE)method and reverse transcription(RT)-PCR with designed degenerated primers.The full-length cDNA sequence of MiBCCP1 was composed of 1 267 bp,comprising a 44-bp 5'-untranslated region(UTR)and a 524-bp 3'-UTR with a typical poly A tail.The open reading frame(ORF)was 699 bp in length encoding a 232-amino-acid protein.After restriction of a 45-amino-acid chloroplast signal peptide,the putative molecular weight of mature MiBCCP1 was about 20 ku.The coding sequence of MiBCCP2 gene was also cloned and it was 789 bp in length encoding a 263-amino-acid protein with a 49-amino-acid chloroplast signal peptide,too.The putative molecular weight of mature MiBCCP2 was approximately 22 ku but it lacked a characteristic biotinylation domain at the C-terminus.Neighbor-joining(NJ)dendrogram showed that these two cloned genes belonged to BCCP1 and BCCP2 clades,which was supported by the bootstrip value of 100.Semi-quantitative reverse transcription(RT)-PCR result showed that during the stress course of nitrogen starvation,the relative transcription of both MiBCCP1 and MiBCCP2 genes increased temporally possibly due to shock,and subsequently declined to the end of this experiment,suggesting that M.incisa had a trend to a declined ability of fatty acid biosynthesis de novo.Taking the significantly increased ArA level in this microalga during this culture course together,it was supposed that the homomeric ACCase in the cytosol playsed a more important role than the heteromeric one in modulating biosynthesis and accumulation of ArA in M.incisa under the nitrogen starvation stress.