Background Phenylpropanoids represent a diverse class of industrially important secondary metabolites, synthesized in plants from phenylalanine and tyrosine. Cyanobacteria have a great potential for sustainable production of phenylpropanoids directly from CO2, due to their photosynthetic lifestyle with a fast growth compared to plants, and the ease of generating genetically engineered strains. This study focuses on photosynthetic production of the first compounds in the phenylpropanoid pathway,

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... trans-cinnamic acid and p-coumaric acid, in the unicellular cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis). This was achieved firstly via heterologous overexpression of a selected set of phenylalanine ammonia lyase (PAL) enzymes from different organisms in Synechocystis. The resulting strains were evaluated for productivity to find the best performing candidate. Secondly, in order to further improve the titer of target compounds, we evaluated the use of stronger expression cassettes for increasing PAL protein levels, as well as knock-out of the laccase gene slr1573, as this was previously reported to prevent degradation of the target compounds in the cell. Finally, to investigate the effect of growth conditions on the production of trans-cinnamic acid and p-coumaric acid from Synechocystis, cultivation conditions promoting rapid, high density growth were tested. Results Results of comparative expression of PALs showed that the highest specific titer was achieved for the strain AtC, expressing a PAL from Arabidopsis thaliana, while a subsequent increase of protein level did not improve the productivity. In contrast to previous reports, the production of target compounds in strains where the slr1573 laccase had been knocked out was found to be lower compared to strains with wild type background. Additionally, the Δslr1573 strains exhibited a strong phenotype of slower growth rate and lower pigment content. The application of a high-density cultivation system for the growth of production strains allowed reaching the highest total titers of trans-cinnamic acid and p-coumaric acid reported so far, at around 0.8 and 0.4 g/L, respectively, after 4 days. Conclusions The production of trans-cinnamic acid, unlike that of p-coumaric acid, is not limited by the protein level of heterologously expressed PAL in Synechocystis. High density cultivation led to higher titres of both products, while knocking out slr1573 did not have a positive effect on production. This work contributes to capability of exploiting the primary metabolism of cyanobacteria for sustainable production of plant phenylpropanoids.