Alkane biosynthesis gene expression and its increased production in recombinant cyanobacteria

Microalgae such as cyanobacteria convert CO₂ to compatible drop-in fuels, such as alkanes. However, the production yield is approximately 0.05-1.0% of the dry weight of natural algae. Here, we aimed to study the role of transcriptional expression, mRNA molecular structure and culture-dependent accumulation of alkanes from two cyanobacteria species. The transcription start sites of the alkane biosynthesis genes ado and aar were identified in the representative cyanobacteria strains Synechocystis sp. PCC 6803 and Limnothrix sp. SK1-2-1, which produce heptadecane and pentadecane, respectively. This characterisation revealed the potential promoters and unique mRNA structures of the ado and aar genes in these species. Transcripts from these genes were induced more in the nitrogen-depleted BG11 (BG11-N) culture than in the BG11 culture, although the biomass was reduced, and as such the amount of alkanes obtained per unit medium was greater for BG11 than for BG11-N. PCC 6803 transconjugants carrying alkane biosynthesis genes from PCC 6803 or SK1-2-1 showed an approximately 1.8- to 2.3-fold increase in heptadecane production compared to the control strain when grown on BG11 cultures without any nitrogen depletion. These results suggest that not only the enzymes ADO/AAR but also the intracellular production of fatty acyl-ACP substrates may be important for the mass production of target alkanes.