Inactivation of phosphate regulator (SphU) in cyanobacterium Synechocystis sp. 6803 directly induced acetyl phosphate pathway leading to enhanced PHB level under nitrogen-sufficient condition

Poly-beta-hydroxybutyrate (PHB) is a biodegradable and biocompatible polymer that has potential in the fields of environmental, agricultural, and biomedical sciences. Cyanobacteria are considered an excellent source of PHB by bioconversion of CO2. This study aimed to prolong PHB production under nitrogen-sufficient condition in the model cyanobacterium Synechocystis sp. PCC 6803. Interestingly, the lack of phosphate regulator (SphU) enabled the mutant strain (Delta SphU) to have the ability to accumulate phosphate with higher expression of Pho regulon. When strain Delta SphU was cultured in nitrogen complete medium for 14 days, the PHB granules were more extensively accumulated in the Delta SphU strain than in the wild type. Photosynthesis activity slightly increased in Delta SphU strain, with no significant difference in chlorophyll a content between wild-type and Delta SphU strain in nitrogen-containing medium, indicating that the higher PHB content (14.57% (w/w) cell dry weight) was not influent of chlorosis. The RT-qPCR analysis revealed that genes involved in PHB biosynthesis and acetyl phosphate pathway were more upregulated in Delta SphU strain. Moreover, the level of acetate production in Delta SphU cells was higher than that in the wild type, suggesting that the deletion of the phosphate regulator could directly induce PHB metabolism by activation of the acetyl phosphate pathway. This research provides better understanding of PHB production regulation in cyanobacteria which are a promising hosts for industrial production of biodegradable plastics.

Automated summary

This study demonstrates that the lack of phosphate regulator (SphU) enables the mutant strain to accumulate more PHB, leading to higher PHB content and production efficiency in cyanobacteria.