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L-leucine is an essential branched chain amino acid (BCAA) used in food, pharmaceutical and animal feed industries. Due to the extensive areas of use, there is an increasing demand on sustainable and economically feasible production methods of L-leucine and other BCAAs. Using photosynthetic organisms for bio-production is advantageous, since they can convert atmospheric carbon dioxide into desired compounds using light as the energy source. Synechocystis sp. PCC 6803 is a unicellular cyanobacterium that naturally possess a biosynthesis pathway for L-leucine production. The regulation mechanisms controlling this pathway are not fully understood, but previous results hint toward a negative feedback regulation at transcriptional and enzymatic levels. In this project, a CRISPR-Cas12 system was employed to engineer promoter regions of leu genes to potentially increase the leucine production and to gain a better understanding of the regulatory mechanisms controlling leucine biosynthesis. Also, genes encoding the transport system for neutral amino acids were deleted to abolish leucine uptake from the extracellular medium. Results indicate that disruption of the transport system increases tolerance to external leucine in high concentrations. At this time no results regarding the leucine production in mutant leu strains have been obtained, but these will hopefully help us better understand how the leucine biosynthesis is regulated, and how leucine production can be improved using metabolic engineering strategies.