Application of a KDPG-aldolase gene-dependent addiction system for enhanced production of cyanophycin in Ralstonia eutropha strain H16.

Two different recombinant plasmids both containing the cyanophycin synthetase gene (cphA) of Synechocystis sp. strain PCC6308 but differing concerning the resistance marker gene were tested for their suitability to produce high amounts of cyanophycin in recombinant strains of Ralstonia eutropha. Various cultivation experiments at the 30-L scale revealed very low cyanophycin contents of the cells ranging from 4.6% to 6.2% (w/w) of cellular dry weight (CDW) only, most probably because most cells had lost the corresponding plasmid during cultivation. To establish a cost effective and high efficient system for production of cyanophycin at larger scales using recombinant strains of R. eutropha, we applied two strategies: First, we integrated cphA into the dispensable chromosomal l-lactate dehydrogenase gene (ldh) of R. eutropha. Depending on the cultivation conditions used, relatively low cyanophycin contents between 2.2% and 7.7% (w/w) of CDW were reproducibly detected, which might be due to weak expression or low gene dosage in the single cphA copy strain of R. eutropha. In a second strategy we constructed a KDPG-aldolase gene (eda)-dependent addiction system, which combined features of a multi-copy plasmid with stabilized expression of cphA. Flasks experiments revealed that the cells accumulated extraordinarily high amounts of cyanophycin between 26.9% and 40.0% (w/w) of CDW even under cultivation conditions lacking cyanophycin precursor substrates or plasmid stabilizing antibiotics. Cyanophycin contents of up to 40.0% (w/w) of CDW were also obtained at a 30-L scale or a 500-L pilot-plant scale under such non-selective conditions. This demonstrates impressively that the stabilizing effect of the constructed eda-dependent addiction system can be used for production of enhanced amounts of cyanophycin at a larger scale in recombinant strains of R. eutropha.