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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 CO 2 , 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 starting compounds of the phenylpropanoid pathway,  trans -cinnamic acid and  p- coumaric acid, in the unicellular cyanobacterium  Synechocystis  sp. PCC 6803 ( Synechocystis ). Results A selected set of phenylalanine ammonia lyase (PAL) enzymes from different organisms was overexpressed in  Synechocystis , and the productivities of the resulting strains compared. 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 and  p- coumaric acids from  Synechocystis , cultivation conditions promoting rapid, high density growth were tested. Comparing the different PALs, the highest specific titer was achieved for the strain AtC, expressing PAL from  Arabidopsis thaliana . A subsequent increase of protein level did not improve the productivity. 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, and the Δ slr1573  strains exhibited a strong phenotype of slower growth rate and lower pigment content. Application of a high-density cultivation system for the growth of production strains allowed reaching the highest total titers of  trans -cinnamic and  p- coumaric acids reported so far, at around 0.8 and 0.4 g L −1 , respectively, after 4 days. Conclusions 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.

Expression of phenylalanine ammonia lyases in Synechocystis sp. PCC 6803 and subsequent improvements of sustainable production of phenylpropanoids