Contribution of protein synthesis depression to poly-β-hydroxybutyrate accumulation in Synechocystis sp. PCC 6803 under nutrient-starved conditions

Preferential phosphatidylglycerol synthesis via phosphorus supply through rRNA degradation in the cyanobacterium, Synechocystis sp. PCC 6803, under phosphate-starved conditions

Photosynthetic organisms often encounter phosphorus (P) limitation in natural habitats. When faced with P limitation, seed plants degrade nucleic acids and extra-plastid phospholipids to remobilize P, thereby enhancing their internal-P utilization efficiency. Although prokaryotic and eukaryotic photosynthetic organisms decrease the content of phosphatidylglycerol (PG) under P-limited conditions, it remains unclear whether PG is degraded for P remobilization. Moreover, information is limited on internal-P remobilization in photosynthetic microbes. This study investigates internal-P remobilization under P-starvation (-P) conditions in a cyanobacterium, Synechocystis sp. PCC 6803, focusing on PG and nucleic acids. Our results reveal that the PG content increases by more than double in the -P culture, indicating preferential PG synthesis among cellular P compounds. Simultaneously, the faster increases of glycolipids counteract this PG increase, which decreases the PG proportion in total lipids. Two genes, glpD and plsX, contribute to the synthesis of diacylglycerol moieties in glycerolipids, with glpD also responsible for the polar head group synthesis in PG. The mRNA levels of both glpD and plsX are upregulated during -P, which would cause the preferential metabolic flow of their P-containing substrates toward glycerolipid synthesis, particularly PG synthesis. Meanwhile, we find that RNA accounts for 62% of cellular P, and that rRNA species, which makes up the majority of RNA, are degraded under -P conditions to less than 30% of their initial levels. These findings emphasize the importance of PG in -P-acclimating cell growth and the role of rRNA as a significant internal-P source for P remobilization, including preferential PG synthesis.

slr2103, a homolog of type-2 diacylglycerol acyltransferase genes, for plastoquinone-related neutral lipid synthesis and NaCl-stress acclimatization in a cyanobacterium, Synechocystis sp. PCC 6803

A cyanobacterium, Synechocystis sp. PCC 6803, contains a lipid with triacylglycerol-like TLC mobility but its identity and physiological roles remain unknown. Here, on ESI-positive LC-MS² analysis, it is shown that the triacylglycerol-like lipid (lipid X) is related to plastoquinone and can be grouped into two subclasses, X_a and X_b, the latter of which is esterified by 16:0 and 18:0. This study further shows that a Synechocystis homolog of type-2 diacylglycerol acyltransferase genes, slr2103, is essential for lipid X synthesis: lipid X disappears in a Synechocystis slr2103-disruptant whereas it appears in an slr2103-overexpressing transformant (OE) of Synechococcus elongatus PCC 7942 that intrinsically lacks lipid X. The slr2103 disruption causes Synechocystis cells to accumulate plastoquinone-C at an abnormally high level whereas slr2103 overexpression in Synechococcus causes the cells to almost completely lose it. It is thus deduced that slr2103 encodes a novel acyltransferase that esterifies 16:0 or 18:0 with plastoquinone-C for the synthesis of lipid X_b. Characterization of the slr2103-disruptant in Synechocystis shows that slr2103 contributes to sedimented-cell growth in a static culture, and to bloom-like structure formation and its expansion by promoting cell aggregation and floatation upon imposition of saline stress (0.3-0.6 M NaCl). These observations provide a basis for elucidation of the molecular mechanism of a novel cyanobacterial strategy to acclimatize to saline stress, and one for development of a system of seawater-utilization and economical harvesting of cyanobacterial cells with high-value added compounds, or blooming control of toxic cyanobacteria.

Chemicals Affecting Cyanobacterial Poly(3-hydroxybutyrate) Accumulation: 2-Phenylethanol Treatment Combined with Nitrogen Deprivation Synergistically Enhanced Poly(3-hydroxybutyrate) Storage in Synechocystis sp. PCC6803 and Anabaena sp. TISTR8076

Various photoautotrophic cyanobacteria increase the accumulation of bioplastic poly(3-hydroxybutyrate) (PHB) under nitrogen deprivation (-N) for energy storage. Several metabolic engineering enhanced cyanobacterial PHB accumulation, but these strategies are not applicable in non-gene-transformable strains. Alternatively, stimulating PHB levels by chemical exposure is desirable because it might be applied to various cyanobacterial strains. However, the study of such chemicals is still limited. Here, 19 compounds previously reported to affect bacterial cellular processes were evaluated for their effect on PHB accumulation in Synechocystis sp. PCC6803, where 3-(3,4-dichlorophenyl)-1,1-dimethylurea, methyl viologen, arsenite, phenoxyethanol and 2-phenylethanol were found to increase PHB accumulation. When cultivated with optimal nitrate supply, Synechocystis contained less than 0.5% [w/w dry weight (DW)] PHB, while cultivation under -N conditions increased the PHB content to 7% (w/w DW). Interestingly, the -N cultivation combined with 2-phenylethanol exposure reduced the Synechocystis protein content by 27% (w/w DW) but significantly increased PHB levels up to 33% (w/w DW), the highest ever reported photoautotrophic cyanobacterial PHB accumulation in a wild-type strain. Results from transcriptomic and metabolomic analysis suggested that under 2-phenylethanol treatment, Synechocystis proteins were degraded to amino acids, which might be subsequently utilized as the source of carbon and energy for PHB biosynthesis. 2-Phenylethanol treatment also increased the levels of metabolites required for Synechocystis PHB synthesis (acetyl-CoA, acetoacetyl-CoA, 3-hydroxybutyryl-CoA and NADPH). Additionally, under -N, the exposure to phenoxyethanol and 2-phenylethanol increased the PHB levels of Anabaena sp. from 0.4% to 4.1% and 6.6% (w/w DW), respectively. The chemicals identified in this study might be applicable for enhancing PHB accumulation in other cyanobacteria.

Disruption of Hydrogen Gas Synthesis Enhances the Cellular Levels of NAD(P)H, Glycogen, Poly(3-hydroxybutyrate) and Photosynthetic Pigments Under Specific Nutrient Condition(s) in Cyanobacterium Synechocystis sp. PCC 6803

In photoautotrophic Synechocystis sp. PCC 6803, NADPH is generated from photosynthesis and utilized in various metabolism, including the biosynthesis of glyceraldehyde 3-phosphate (the upstream substrate for carbon metabolism), poly(3-hydroxybutyrate) (PHB), photosynthetic pigments, and hydrogen gas (H2). Redirecting NADPH flow from one biosynthesis pathway to another has yet to be studied. Synechocystis's H2 synthesis, one of the pathways consuming NAD(P)H, was disrupted by the inactivation of hoxY and hoxH genes encoding the two catalytic subunits of hydrogenase. Such inactivation with a complete disruption of H2 synthesis led to 1.4-, 1.9-, and 2.1-fold increased cellular NAD(P)H levels when cells were cultured in normal medium (BG11), the medium without nitrate (-N), and the medium without phosphate (-P), respectively. After 49-52 d of cultivation in BG11 (when the nitrogen source in the media was depleted), the cells with disrupted H2 synthesis had 1.3-fold increased glycogen level compared to wild type of 83-85% (w/w dry weight), the highest level reported for cyanobacterial glycogen. The increased glycogen content observed by transmission electron microscopy was correlated with the increased levels of glucose 6-phosphate and glucose 1-phosphate, the two substrates in glycogen synthesis. Disrupted H2 synthesis also enhanced PHB accumulation up to 1.4-fold under -P and 1.6-fold under -N and increased levels of photosynthetic pigments (chlorophyll a, phycocyanin, and allophycocyanin) by 1.3- to 1.5-fold under BG11. Thus, disrupted H2 synthesis increased levels of NAD(P)H, which may be utilized for the biosynthesis of glycogen, PHB, and pigments. This strategy might be applicable for enhancing other biosynthetic pathways that utilize NAD(P)H.

Cyanobacterium Synechocystis sp. PCC 6803 lacking adc1 gene produces higher polyhydroxybutyrate accumulation under modified nutrients of acetate supplementation and nitrogen-phosphorus starvation

Increased polyhydroxybutyrate production in cyanobacterium Synechocystis sp. PCC 6803 lacking adc1 gene (Δadc1) is first-timely reported in this study. We constructed the mutant by disrupting adc1 gene encoding arginine decarboxylase, thereby exhibiting a partial blockade of polyamine synthesis. This Δadc1 mutant had a proliferative growth and certain contents of intracellular pigments including chlorophyll a and carotenoids as similar as those of wild type (WT). Highest PHB production was certainly induced by BG₁₁-N-P+A condition in both WT and Δadc1 mutant of about 24.9 %w/DCW at day 9 and 36.1 %w/DCW at day 7 of adaptation time, respectively. Abundant PHB granules were also visualized under both BG₁₁-N-P and BG₁₁-N-P+A conditions. All pha transcript amounts of Δadc1 mutant grown at 7 days-adaptation time were clearly upregulated corresponding to its PHB content under BG₁₁-N-P+A condition. Our finding indicated that this adc1 perturbation is alternatively achieved for PHB production in Synechocystis sp. PCC 6803.

Modifying the Cyanobacterial Metabolism as a Key to Efficient Biopolymer Production in Photosynthetic Microorganisms

Cyanobacteria are photoautotrophic bacteria commonly found in the natural environment. Due to the ecological benefits associated with the assimilation of carbon dioxide from the atmosphere and utilization of light energy, they are attractive hosts in a growing number of biotechnological processes. Biopolymer production is arguably one of the most critical areas where the transition from fossil-derived chemistry to renewable chemistry is needed. Cyanobacteria can produce several polymeric compounds with high applicability such as glycogen, polyhydroxyalkanoates, or extracellular polymeric substances. These important biopolymers are synthesized using precursors derived from central carbon metabolism, including the tricarboxylic acid cycle. Due to their unique metabolic properties, i.e., light harvesting and carbon fixation, the molecular and genetic aspects of polymer biosynthesis and their relationship with central carbon metabolism are somehow different from those found in heterotrophic microorganisms. A greater understanding of the processes involved in cyanobacterial metabolism is still required to produce these molecules more efficiently. This review presents the current state of the art in the engineering of cyanobacterial metabolism for the efficient production of these biopolymers.