Synechocystis mutants defective in manganese uptake regulatory system, ManSR, are hypersensitive to strong light

Coordination of carbon partitioning and photosynthesis by a two-component signaling network in Synechococcus elongatus PCC 7942

Photosynthetic organisms need to balance the rate of photosynthesis with the utilization of photosynthetic products by downstream reactions. While such "source/sink" pathways are well-interrogated in plants, analogous regulatory systems are unknown or poorly studied in single-celled algal and cyanobacterial species. Towards the identification of energy/sugar sensors in cyanobacteria, we utilized an engineered strain of Synechococcus elongatus PCC 7942 that allows experimental manipulation of carbon status. We conducted a screening of all two-component systems (TCS) and serine/threonine kinases (STKs) encoded in S. elongatus PCC 7942 by analyzing phenotypes consistent with sucrose-induced relaxation of sink inhibition. We narrowed the candidate sensor proteins by analyzing changes observed after sucrose feeding. We show that a clustered TCS network containing RpaA, CikB, ManS and NblS are involved in the regulation of genes related to photosynthesis, pigment synthesis, and Rubisco concentration in response to sucrose. Altogether, these results highlight a regulatory TCS group that may play under-appreciated functions in carbon partitioning and energy balancing in cyanobacteria.

Cyanobacteria as cell factories for the photosynthetic production of sucrose

Biofuels and other biologically manufactured sustainable goods are growing in popularity and demand. Carbohydrate feedstocks required for industrial fermentation processes have traditionally been supplied by plant biomass, but the large quantities required to produce replacement commodity products may prevent the long-term feasibility of this approach without alternative strategies to produce sugar feedstocks. Cyanobacteria are under consideration as potential candidates for sustainable production of carbohydrate feedstocks, with potentially lower land and water requirements relative to plants. Several cyanobacterial strains have been genetically engineered to export significant quantities of sugars, especially sucrose. Sucrose is not only naturally synthesized and accumulated by cyanobacteria as a compatible solute to tolerate high salt environments, but also an easily fermentable disaccharide used by many heterotrophic bacteria as a carbon source. In this review, we provide a comprehensive summary of the current knowledge of the endogenous cyanobacterial sucrose synthesis and degradation pathways. We also summarize genetic modifications that have been found to increase sucrose production and secretion. Finally, we consider the current state of synthetic microbial consortia that rely on sugar-secreting cyanobacterial strains, which are co-cultivated alongside heterotrophic microbes able to directly convert the sugars into higher-value compounds (e.g., polyhydroxybutyrates, 3-hydroxypropionic acid, or dyes) in a single-pot reaction. We summarize recent advances reported in such cyanobacteria/heterotroph co-cultivation strategies and provide a perspective on future developments that are likely required to realize their bioindustrial potential.

A transporter Slr1512 involved in bicarbonate and pH-dependent acclimation mechanism to high light stress in Synechocystis sp. PCC 6803

High light (HL) exposure leads to photoinhibition and excess accumulation of toxic reactive oxygen species (ROS) in photosynthetic organisms, negatively impacting the global primary production. In this study, by screening a mutant library, a gene related with bicarbonate transport, slr1512, was found involved in HL acclimation in model cyanobacterium Synechocystis sp. PCC 6803. Comparative growth analysis showed that the slr1512 knockout mutant dramatically enhanced the tolerance of Synechocystis towards long-term HL stress (200 μmol photons m^-2 s^-1) than the wild type, achieving an enhanced growth by ~1.95-folds after 10 d. The phenotype differences between Δslr1512 and the wild type were analyzed via absorption spectrum and chlorophyll a content measurement. In addition, the accessible bicarbonate controlled by slr1512 and decreased PSII activity were demonstrated, and they were found to be the key factors affecting the tolerance of Synechocystis against HL stress. Further analysis confirmed that intracellular bicarbonate can significantly affect the activity of photosystem II, leading to the altered accumulation of toxic ROS under HL. Finally, a comparative transcriptomics was applied to determine the differential responses to HL between Δslr1512 and the wild type. This work provides useful insights to long-term acclimation mechanisms towards HL and valuable information to guide the future tolerance engineering of cyanobacteria against HL.

From the Uncharacterized Protein Family 0016 to the GDT1 family: Molecular insights into a newly-characterized family of cation secondary transporters

The Uncharacterized Protein Family 0016 (UPF0016) gathers poorly studied membrane proteins well conserved through evolution that possess one or two copies of the consensus motif Glu-x-Gly-Asp-(Arg/Lys)-(Ser/Thr). Members are found in many eukaryotes, bacteria and archaea. The interest for this protein family arose in 2012 when its human member TMEM165 was linked to the occurrence of Congenital Disorders of Glycosylation (CDGs) when harbouring specific mutations. Study of the UPF0016 family is undergone through the characterization of the bacterium Vibrio cholerae (MneA), cyanobacterium Synechocystis (SynPAM71), yeast Saccharomyces cerevisiae (Gdt1p), plant Arabidopsis thaliana (PAM71 and CMT1), and human (TMEM165) members. These proteins have all been identified as transporters of cations, more precisely of Mn²⁺, with an extra reported function in Ca²⁺ and/or H⁺ transport for some of them. Apart from glycosylation in humans, the UPF0016 members are required for lactation in humans, photosynthesis in plants and cyanobacteria, Ca²⁺ signaling in yeast, and Mn²⁺ homeostasis in the five aforementioned species. The requirement of the UPF0016 members for key physiological processes most likely derives from their transport activity at the Golgi membrane in human and yeast, the chloroplasts membranes in plants, the thylakoid and plasma membranes in cyanobacteria, and the cell membrane in bacteria. In the light of these studies on various UPF0016 members, this family is not considered as uncharacterized anymore and has been renamed the Gdt1 family according to the name of its S. cerevisiae member. This review aims at assembling and confronting the current knowledge in order to identify shared and distinct features in terms of transported molecules, mode of action, structure, etc., as well as to better understand their corresponding physiological roles.

Manganese Homeostasis in Cyanobacteria

Manganese (Mn) is essential for life on earth. As a catalyst of the water oxidation reaction within photosystem II, the trace metal is responsible for the evolution of virtually all oxygen in the earth’s atmosphere. Mn acts furthermore as an activator or cofactor of numerous enzymes involved in reactive oxygen species scavenging or central and secondary metabolism. While the sufficient supply of oxygenic photosynthetic organisms with Mn is obvious for maintaining photosynthetic activity, the avoidance of cellular Mn overload is also critical. In this review, current knowledge about the Mn homeostasis network in the model cyanobacterium Synechocystis sp. PCC 6803 is presented, including transporters and regulators.

The transporter SynPAM71 is located in the plasma membrane and thylakoids, and mediates manganese tolerance in Synechocystis PCC6803

Manganese (Mn) is an essential constituent of photosystem II (PSII) and therefore indispensable for oxygenic photosynthesis. Very little is known about how Mn is transported, delivered and retained in photosynthetic cells. Recently, the thylakoid-localized transporter PAM71 has been linked to chloroplast Mn homeostasis in Arabidopsis thaliana. Here, we characterize the function of its homolog in Synechocystis (SynPAM71). We used a loss-of-function line (ΔSynPAM71), wild-type (WT) cells exposed to Mn stress and strains expressing a tagged variant of SynPAM71 to characterize the role of SynPAM71 in cyanobacterial Mn homeostasis. The ΔSynPAM71 strain displays an Mn-sensitive phenotype with reduced levels of chlorophyll and PSI accumulation, defects in PSII photochemistry and intracellular Mn enrichment, particularly in the thylakoid membranes. These effects are attributable to Mn toxicity, as very similar symptoms were observed in WT cells exposed to excess Mn. Moreover, CyanoP, which is involved in the early steps of PSII assembly, is massively upregulated in ΔSynPAM71. SynPAM71 was detected in both the plasma membrane and, to a lesser extent, the thylakoid membranes. Our results suggest that SynPAM71 is involved in the maintenance of Mn homeostasis through the export of Mn from the cytoplasm into the periplasmic and luminal compartments, where it can be stored without interfering with cytoplasmic metabolic processes.