An RpaA-Dependent Sigma Factor Cascade Sets the Timing of Circadian Transcriptional Rhythms in Synechococcus elongatus

Clocking out and letting go to unleash green biotech applications in a photosynthetic host

Cyanobacteria are photosynthetic bacteria whose gene expression patterns are globally regulated by their circadian (daily) clocks. Due to their ability to use sunlight as their energy source, they are also attractive hosts for "green" production of pharmaceuticals, renewable fuels, and chemicals. However, despite the application of traditional genetic tools such as the identification of strong promoters to enhance the expression of heterologous genes, cyanobacteria have lagged behind other microorganisms such as Escherichia coli and yeast as economically efficient cell factories. The previous approaches have ignored large-scale constraints within cyanobacterial metabolic networks on transcription, predominantly the pervasive control of gene expression by the circadian (daily) clock. Here, we show that reprogramming gene expression by releasing circadian repressor elements in the transcriptional regulatory pathways coupled with inactivation of the central oscillating mechanism enables a dramatic enhancement of expression in cyanobacteria of heterologous genes encoding both catalytically active enzymes and polypeptides of biomedical significance.

Tailoring regulatory components for metabolic engineering in cyanobacteria

The looming climate crisis has prompted an ever-growing interest in cyanobacteria due to their potential as sustainable production platforms for the synthesis of energy carriers and value-added chemicals from CO2 and sunlight. Nonetheless, cyanobacteria are yet to compete with heterotrophic systems in terms of space-time yields and consequently production costs. One major drawback leading to the low production performance observed in cyanobacteria is the limited ability to utilize the full capacity of the photosynthetic apparatus and its associated systems, i.e. CO2 fixation and the directly connected metabolism. In this review, novel insights into various levels of metabolic regulation of cyanobacteria are discussed, including the potential of targeting these regulatory mechanisms to create a chassis with a phenotype favorable for photoautotrophic production. Compared to conventional metabolic engineering approaches, minor perturbations of regulatory mechanisms can have wide-ranging effects.

Synechococcus elongatus Argonaute reduces natural transformation efficiency and provides immunity against exogenous plasmids

IMPORTANCE

S. elongatus is an important cyanobacterial model organism for the study of its prokaryotic circadian clock, photosynthesis, and other biological processes. It is also widely used for genetic engineering to produce renewable biochemicals. Our findings reveal an SeAgo-based defense mechanism in S. elongatus against the horizontal transfer of genetic material. We demonstrate that deletion of the ago gene facilitates genetic studies and genetic engineering of S. elongatus.

Synchronization of the circadian clock to the environment tracked in real time

The circadian system of the cyanobacterium Synechococcus elongatus PCC 7942 relies on a three-protein nanomachine (KaiA, KaiB, and KaiC) that undergoes an oscillatory phosphorylation cycle with a period of ~24 h. This core oscillator can be reconstituted in vitro and is used to study the molecular mechanisms of circadian timekeeping and entrainment. Previous studies showed that two key metabolic changes that occur in cells during the transition into darkness, changes in the ATP/ADP ratio and redox status of the quinone pool, are cues that entrain the circadian clock. By changing the ATP/ADP ratio or adding oxidized quinone, one can shift the phase of the phosphorylation cycle of the core oscillator in vitro. However, the in vitro oscillator cannot explain gene expression patterns because the simple mixture lacks the output components that connect the clock to genes. Recently, a high-throughput in vitro system termed the in vitro clock (IVC) that contains both the core oscillator and the output components was developed. Here, we used IVC reactions and performed massively parallel experiments to study entrainment, the synchronization of the clock with the environment, in the presence of output components. Our results indicate that the IVC better explains the in vivo clock-resetting phenotypes of wild-type and mutant strains and that the output components are deeply engaged with the core oscillator, affecting the way input signals entrain the core pacemaker. These findings blur the line between input and output pathways and support our previous demonstration that key output components are fundamental parts of the clock.

Halotolerance, stress mechanisms, and circadian clock of salt-tolerant cyanobacteria

Cyanobacteria harbor a high level of physiological flexibility, which enables them to reside in virtually all available environmental niches, including extreme environments. In this review, we summarize the recent advancements in stress mechanisms of salt-tolerant (a.k.a. halotolerant) cyanobacteria. Omics approaches have been extensively employed in recent years to decipher mechanisms of halotolerance and to understand the relevance of halotolerance-associated gene regulatory networks. The vast knowledge from genome mining disclosed that halotolerant cyanobacteria possess extended gene families and/or clusters, encoding enzymes that synthesize unique osmoprotectants, including glycine betaine (GB), betaine derivatives, and mycosporine-like amino acids (MAAs). Comprehensive transcriptomic analyses were conducted using Halothece sp. PCC7418 (hereafter referred to as Halothece), a cyanobacterium that exhibits remarkable halotolerance. These studies revealed a specific transcriptional response when Halothece was subjected to salt stress, whereas salt and osmotic stresses were found to share a common transcriptomic response. Transcriptome and metabolite analyses of Halothece illustrated a complex dynamic relationship between the biosyntheses of osmoprotectants, as well as corresponding and ancillary pathways. Lastly, novel insights highlight the relationship between the molecular regulation of the circadian rhythm and salt stress tolerance. Since the circadian rhythm of gene expression was distorted under salt stress, halotolerant cyanobacteria may prioritize the adaptation to salt stress by attenuation of circadian rhythmicity. KEY POINTS: • Recent advancements in the understanding of stress mechanisms in halotolerant cyanobacteria are described based on omics analyses. • Transcriptome and metabolite analyses of Halothece illustrated a complex dynamic relationship between the biosyntheses of osmoprotectants, as well as corresponding and ancillary pathways. • Since salt stress affects the molecular regulation among clock-related proteins, salt stress may attenuate circadian rhythmicity.

Thermodynamics contributes to high limonene productivity in cyanobacteria

Terpenoids are a large group of secondary metabolites with broad industrial applications. Engineering cyanobacteria is an attractive route for the sustainable production of commodity terpenoids. Currently, a major obstacle lies in the low productivity attained in engineered cyanobacterial strains. Traditional metabolic engineering to improve pathway kinetics has led to limited success in enhancing terpenoid productivity. In this study, we reveal thermodynamics as the main determinant for high limonene productivity in cyanobacteria. Through overexpressing the primary sigma factor, a higher photosynthetic rate was achieved in an engineered strain of Synechococcus elongatus PCC 7942. Computational modeling and wet lab analyses showed an increased flux toward both native carbon sink glycogen synthesis and the non-native limonene synthesis from photosynthate output. On the other hand, comparative proteomics showed decreased expression of terpene pathway enzymes, revealing their limited role in determining terpene flux. Lastly, growth optimization by enhancing photosynthesis has led to a limonene titer of 19 mg/L in 7 days with a maximum productivity of 4.3 mg/L/day. This study highlights the importance of enhancing photosynthesis and substrate input for the high productivity of secondary metabolic pathways, providing a new strategy for future terpenoid engineering in phototrophs.

•

Pathway enzyme engineering marginally increases cyanobacterial terpene production.

•

Sigma factor overexpression improves photosynthetic efficiency in cyanobacteria.

•

Enhanced photosynthesis results in high limonene production in cyanobacteria.

•

Enhanced photosynthesis provides high thermodynamic driving force for terpenes.

Approaches in the photosynthetic production of sustainable fuels by cyanobacteria using tools of synthetic biology

Cyanobacteria, photosynthetic prokaryotic microorganisms having a simple genetic composition are the prospective photoautotrophic cell factories for the production of a wide range of biofuel molecules. The simple genetic composition of cyanobacteria allows effortless genetic manipulation which leads to increased research endeavors from the synthetic biology approach. Various unicellular model cyanobacterial strains like Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 have been successfully engineered for biofuels generation. Improved development of synthetic biology tools, genetic modification methods and advancement in transformation techniques to construct a strain that can contain multiple foreign genes in a single operon have vastly expanded the functions that can be used for engineering photosynthetic cyanobacteria for the generation of various biofuel molecules. In this review, recent advancements and approaches in synthetic biology tools used for cyanobacterial genome editing have been discussed. Apart from this, cyanobacterial productions of various fuel molecules like isoprene, limonene, α-farnesene, squalene, alkanes, butanol, and fatty acids, which can be a substitute for petroleum and fossil fuels in the future, have been elaborated.

The Circadian Clock-A Molecular Tool for Survival in Cyanobacteria

Cyanobacteria are photosynthetic organisms that are known to be responsible for oxygenating Earth’s early atmosphere. Having evolved to ensure optimal survival in the periodic light/dark cycle on this planet, their genetic codes are packed with various tools, including a sophisticated biological timekeeping system. Among the cyanobacteria is Synechococcus elongatus PCC 7942, the simplest clock-harboring organism with a powerful genetic tool that enabled the identification of its intricate timekeeping mechanism. The three central oscillator proteins—KaiA, KaiB, and KaiC—drive the 24 h cyclic gene expression rhythm of cyanobacteria, and the “ticking” of the oscillator can be reconstituted inside a test tube just by mixing the three recombinant proteins with ATP and Mg²⁺. Along with its biochemical resilience, the post-translational rhythm of the oscillation can be reset through sensing oxidized quinone, a metabolite that becomes abundant at the onset of darkness. In addition, the output components pick up the information from the central oscillator, tuning the physiological and behavioral patterns and enabling the organism to better cope with the cyclic environmental conditions. In this review, we highlight our understanding of the cyanobacterial circadian clock and discuss how it functions as a molecular chronometer that readies the host for predictable changes in its surroundings.

Sigma Factor Modulation for Cyanobacterial Metabolic Engineering

Sigma (σ) factors are key regulatory proteins that control the transcription initiation in prokaryotes. In response to environmental or developmental cues, σ factors initiate the transcription of necessary genes responsible for maintaining a life-sustaining metabolic balance. Due to the significant role of σ factors in bacterial metabolism, their rational engineering for commercial metabolite production in photoautotrophic, cyanobacterial cells is a desirable venture. As cyanobacterial genomes typically encode multiple σ factors, effective execution of metabolic engineering efforts largely relies on uncovering the complicated gene regulatory network and further characterization of the members of σ factor regulatory circuits. This review outlines the prospects of σ factor in metabolic engineering of cyanobacteria, summarizes the challenges in the path towards an efficient strain construction and highlights the genomic context of putative regulators of cyanobacterial σ factors.

Safety of photosynthetic Synechococcus elongatus for in vivo cyanobacteria-mammalian symbiotic therapeutics

The cyanobacterium Synechococcus elongatus (SE) has been shown to rescue ischaemic heart muscle after myocardial infarction by photosynthetic oxygen production. Here, we investigated SE toxicity and hypothesized that systemic SE exposure does not elicit a significant immune response in rats. Wistar rats intravenously received SE (n = 12), sterile saline (n = 12) or E. coli lipopolysaccharide (LPS, n = 4), and a subset (8 SE, 8 saline) received a repeat injection 4 weeks later. At baseline, 4 h, 24 h, 48 h, 8 days and 4 weeks after injection, clinical assessments, blood cultures, blood counts, lymphocyte phenotypes, liver function tests, proinflammatory cytokines and immunoglobulins were assessed. Across all metrics, SE rats responded comparably to saline controls, displaying no clinically significant immune response. As expected, LPS rats exhibited severe immunological responses. Systemic SE administration does not induce sepsis or toxicity in rats, thereby supporting the safety of cyanobacteria-mammalian symbiotic therapeutics using this organism.

Dynamic Inventory of Intermediate Metabolites of Cyanobacteria in a Diurnal Cycle

Cyanobacteria are gaining importance both as hosts for photoautotrophic production of chemicals and as model systems for studies of diurnal lifestyle. The proteome and transcriptome of cyanobacteria have been closely examined under diurnal growth, whereas the downstream effects on the intermediary metabolism have not received sufficient attention. The present study focuses on identifying the cellular metabolites whose inventories undergo dramatic changes in a fast-growing cyanobacterium, Synechococcus elongatus PCC 11801. We identified and quantified 67 polar metabolites, whose inventory changes significantly during diurnal growth, with some metabolites changing by 100-fold. The Calvin-Benson-Bassham cycle intermediates peak at midday to support fast growth. The hitherto unexplored γ-glutamyl peptides act as reservoirs of amino acids. Interestingly, several storage molecules or their precursors accumulate during the dark phase, dispelling the notion that all biosynthetic activity takes place in the light phase. Our results will guide metabolic modeling and strain engineering of cyanobacteria.

•

We identify and quantify 67 polar intermediate metabolites in cyanobacteria via LC-MS

•

A number of metabolites show large variations during the diurnal cycle

•

Intermediates of the CBB cycle peak at midday, coinciding with peak in growth rate

•

Gamma-glutamyl dipeptides identified as new storage compounds that peak at dawn

Tightening the Screws on PsbA in Cyanobacteria

Cyanobacterial genomes encode several isoforms of the D1 (PsbA) subunit of Photosystem II (PSII). The distinct regulation of each isoform ensures adaptation under changing environmental conditions. Uncovering the missing elements of signal transduction pathways and psbA gene expression could open new avenues in engineering programs of cyanobacterial strains.

The circadian clock and darkness control natural competence in cyanobacteria

The cyanobacterium Synechococcus elongatus is a model organism for the study of circadian rhythms. It is naturally competent for transformation—that is, it takes up DNA from the environment, but the underlying mechanisms are unclear. Here, we use a genome-wide screen to identify genes required for natural transformation in S. elongatus, including genes encoding a conserved Type IV pilus, genes known to be associated with competence in other bacteria, and others. Pilus biogenesis occurs daily in the morning, while natural transformation is maximal when the onset of darkness coincides with the dusk circadian peak. Thus, the competence state in cyanobacteria is regulated by the circadian clock and can adapt to seasonal changes of day length.

The cyanobacterium Synechococcus elongatus is a model organism for the study of circadian rhythms, and is naturally competent for transformation. Here, Taton et al. identify genes required for natural transformation in this organism, and show that the coincidence of circadian dusk and darkness regulates the competence state in different day lengths.