Synechocystis sp. PCC 6803 with ddh disruption and heterologous pk gene expression in the phosphoketolase pathway drives carbon flux toward polyhydroxybutyrate production

Abstract

Background

To overcome the limitations in the production of polyhydroxybutyrate (PHB) in cyanobacteria, we genetically modified Synechocystis sp. PCC 6803 focusing on carbon metabolism at the phosphoketolase pathway. In addition, for the baseline of the control strain (Ct), an inhibition of the pyruvate-to-lactate reaction was created by the disruption of the native ddh gene that encodes lactate dehydrogenase.

Results

The effects on cell growth of phosphoketolase (pk) gene expression derived from Pseudomonas aeruginosa ATCC 15442 (Ct_OXpkPa), Bifidobacterium breve strain 203 (Ct_OXpkBb), and Bifidobacterium adolescentis ATCC 15703 (Ct_OXpkBa) were comparable to that observed in the Ct strain. Notably, the PHB production in the Ct_OXpkBb strain increased under the normal growth condition without any stress, reaching 32.5% of dry cell weight. Combined nitrogen and phosphorus deprivation, the Ct_OXpkBb strain significantly accumulated PHB up to 62.2% of dry cell weight within 7 days of treatment. Compared with the Ct strain, the Ct_OXpkBb strain also exhibited a significant reduction in glycogen, in accordance with decreased glgC transcript levels for glycogen synthesis and increased glgX transcript levels for glycogen degradation. Furthermore, under the NP-deprived condition, the Ct_OXpkBb strain exhibited reduced transcript levels of acetate metabolism genes, in particular ackA and acs, compared with the control condition.

Conclusions

Synechocystis PCC 6803 expressing pk from Bifidobacterium breve strain 203 resulted in significant production of PHB, indicating its potential for biotechnological applications.