Expansion of bilin-based red light sensors in the subaerial desert cyanobacterium Nostoc flagelliforme

Red-light signaling pathway activates desert cyanobacteria to prepare for desiccation tolerance

Desiccation-tolerant cyanobacteria are able to survive frequent cycles of hydration and dehydration, which are closely linked to diurnal light oscillations. Previous studies have shown that light serves as a crucial anticipatory cue, activating desert cyanobacteria to prepare for desiccation. However, the mechanisms underlying their light-coupled desiccation tolerance remain largely unknown. Here, we demonstrate that red-light-induced photosynthetic genes are positively regulated by a LuxR family transcription factor NfSrr1. We further identified the cyanobacteriochrome NfPixJ as interacting with NfSrr1 and functioning as a red light sensor. Phenotypic analysis revealed that the red-light signaling module NfPixJ-NfSrr1 plays a key role in preparing cyanobacteria for desiccation tolerance. This module also regulates the synthesis of protective compatible solutes, suggesting that red light functions as a global regulatory signal for the broader stress response. Phylogenetic analysis indicates that the presence of this red-light signaling pathway, mediated by NfPixJ-NfSrr1 module, correlates with the ability of cyanobacteria to thrive in water-deficit habitats. Overall, our findings uncover a red-light signaling pathway that enhances desiccation tolerance as desert cyanobacteria encounter red light at dawn, before water limitation. These results provide insights into the mechanisms behind light-induced anticipatory stress tolerance in photosynthetic organisms.

Red-light-dependent chlorophyll synthesis kindles photosynthetic recovery of chlorotic dormant cyanobacteria using a dark-operative enzyme

Chlorosis dormancy resulting from nitrogen starvation and its resuscitation upon available nitrogen contributes greatly to the fitness of cyanobacterial population under nitrogen-fluctuating environments. The reinstallation of the photosynthetic machinery is a key process for resuscitation from a chlorotic dormant state; however, the underlying regulatory mechanism is still elusive. Here, we reported that red light is essential for re-greening chlorotic Synechocystis sp. PCC 6803 (a non-diazotrophic cyanobacterium) after nitrogen supplement under weak light conditions. The expression of dark-operative protochlorophyllide reductase (DPOR) governed by the transcriptional factor RpaB was strikingly induced by red light in chlorotic cells, and its deficient mutant lost the capability of resuscitation from a dormant state, indicating DPOR catalyzing chlorophyll synthesis is a key step in the photosynthetic recovery of dormant cyanobacteria. Although light-dependent protochlorophyllide reductase is widely considered as a master switch in photomorphogenesis, this study unravels the primitive DPOR as a spark to activate the photosynthetic recovery of chlorotic dormant cyanobacteria. These findings provide new insight into the biological significance of DPOR in cyanobacteria and even some plants thriving in extreme environments.

Fluorescence capturing behaviour of cyanobacterial resilience: Insights into UV-exposed ecosystems and its environmental applications

Cyanobacteria are resilient microorganisms and thrive in environments exposed to UV radiation, ranging from ocean surfaces to scorching hot springs and dry expanses. 'Cyanobacterial Resilience' refers to their ability to withstand UV radiation, revealing intricate genomic secrets and adaptive mechanisms ensuring survival. These mechanisms include metabolic adaptations, robust DNA repair systems and UV-protective compounds such as Scytonemin and Mycosporine, vital for shielding against UV radiation survival. Cyanobacteria are crucial pioneers in UV-exposed ecosystems, highlighting their resilience and adaptability. Some cyanobacteria exhibit luminescence, emitting blue-green light due to phycobiliproteins, while bioluminescence in cyanobacteria, if it occurs, involves different compounds rather than luciferins and luciferase enzymes. This luminescence holds promise for various biotechnological applications, such as biosensors, imaging probes and carbon sequestration, for participating in photocatalytic processes for water purification and CO2 conversion, and contributes to solar simulation studies to advance photosynthesis and renewable energy technologies. The versatile applications of these materials highlight their ecological importance and potential in addressing global challenges. In conclusion, 'Cyanobacterial Resilience' highlights the remarkable adaptation strategies of cyanobacteria in UV-exposed environments. It emphasises their role as pioneers and innovators in biological and technological domains, providing insights into their enduring impact on ecosystems and scientific advancement.

Teal-light absorbing cyanobacterial phytochrome superfamily provides insights into the diverse mechanisms of spectral tuning and facilitates the engineering of photoreceptors for optogenetic tools

Cyanobacteriochromes (CBCRs) are distinctive tetrapyrrole (bilin)-binding photoreceptors exclusively found in cyanobacteria. Unlike canonical phytochromes, CBCRs require only a GAF (cGMP-phosphodiesterase/adenylate cyclase/FhlA) domain for autolyase activity to form a bilin adduct via a Cys residue and cis-trans photoisomerization. Apart from the canonical Cys, which attaches covalently to C31 in the A-ring of the bilin, some GAF domains of CBCRs contain a second-Cys in the Asp-Xaa-Cys-Phe (DXCF) motif, responsible for isomerization of phycocyanobilin (PCB) to phycoviolobilin (PVB) and/or for the formation of a reversible 2nd thioether linkage to the C10. Unlike green/teal-absorbing GAF proteins lacking ligation activity, the second-Cys in another teal-absorbing lineage (DXCF blue/teal group) exhibits both isomerization and ligation activity due to the presence of the Tyr instead of His next to the canonical Cys. Herein, we discovered an atypical CBCR GAF protein, Tpl7205g1, belonging to the DXCF blue/teal group, but having His instead of Tyr next to the first-Cys. Consistent with its subfamily, the second-Cys of Tpl7205g1 did not form a thioether linkage at C10 of PCB, showing only isomerization activity. Instead of forming 2nd thioether linkage, this novel GAF protein exhibits a pH-dependent photocycle between protonated 15Z and deprotonated 15E. Site-directed mutagenesis to the GAF scaffolds revealed its combined characteristics, including properties of teal-DXCF CBCRs and red/green-absorbing CBCRs (XRG CBCRs), suggesting itself as the evolutionary bridge between the two CBCR groups. Our study thus sheds light on the expanded spectral tuning characteristics of teal-light absorbing CBCRs and enhances feasibility of engineering these photoreceptors.

Light regulation in critical human pathogens of clinical relevance such as Acinetobacter baumannii, Staphylococcus aureus and Pseudomonas aeruginosa

It is now clearly recognized that light modulates the physiology of many bacterial chemotrophs, either directly or indirectly. An interesting case are bacterial pathogens of clinical relevance. This work summarizes, discusses, and provides novel complementary information to what is currently known about light sensing and responses in critical human pathogens such as Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus aureus. These pathogens are associated with severe hospital and community infections difficult to treat due to resistance to multiple drugs. Moreover, light responses in Brucella abortus, an important animal and human pathogen, are also compiled. Evidence recovered so far indicates that light modulates aspects related to pathogenesis, persistence, and antibiotic susceptibility in these pathogens; such as motility, biofilm formation, iron uptake, tolerance to antibiotics, hemolysis and virulence. The pathogens elicit differential responses to light depending likely on their pathophysiology, ability to cause disease and characteristics of the host. The response to light is not restricted to discrete physiological traits but is global. In higher organisms, light provides spatial and temporal information. Then, it is crucial to understand what information light is providing in these bacterial pathogens. Our current hypothesis postulates that light serves as a signal that allows these pathogens to synchronize their behavior to the circadian rhythm of the host, to optimize infection. Advances on the molecular mechanism of light signal transduction and physiological responses to light, as well as in the relation between light and bacterial infection, would not only enlarge our understanding of bacterial pathogenesis but also could potentially provide alternative treatment options for infectious illnesses.

BfmRS encodes a regulatory system involved in light signal transduction modulating motility and desiccation tolerance in the human pathogen Acinetobacter baumannii

We have previously shown that Acinetobacter baumannii as well as other relevant clinical bacterial pathogens such as Staphylococcus aureus and Pseudomonas aeruginosa, perceive and respond to light at 37 °C, the normal temperature in mammal hosts. In this work, we present evidence indicating that the two-component system BfmRS transduces a light signal in A. baumannii at this temperature, showing selective involvement of the BfmR and BfmS components depending on the specific cellular process. In fact, both BfmR and BfmS participate in modulation of motility by light, while only BfmR is involved in light regulation of desiccation tolerance in this microorganism. Neither BfmR nor BfmS contain a photoreceptor domain and then most likely, the system is sensing light indirectly. Intriguingly, this system inhibits blsA expression at 37 °C, suggesting antagonistic functioning of both signaling systems. Furthermore, we present evidence indicating that the phosphorylatable form of BfmR represses motility. Overall, we provide experimental evidence on a new biological function of this multifaceted system that broadens our understanding of A. baumannii's physiology and responses to light.

Coevolution of tandemly repeated hlips and RpaB-like transcriptional factor confers desiccation tolerance to subaerial Nostoc species

Desert-inhabiting cyanobacteria can tolerate extreme desiccation and quickly revive after rehydration. The regulatory mechanisms that enable their vegetative cells to resurrect upon rehydration are poorly understood. In this study, we identified a single gene family of high light-inducible proteins (Hlips) with dramatic expansion in the Nostoc flagelliforme genome and found an intriguingly special convergence formed through four tandem gene duplication. The emerged four independent hlip genes form a gene cluster (hlips-cluster) and respond to dehydration positively. The gene mutants in N. flagelliforme were successfully generated by using gene-editing technology. Phenotypic analysis showed that the desiccation tolerance of hlips-cluster-deleted mutant decreased significantly due to impaired photosystem II repair, whereas heterologous expression of hlips-cluster from N. flagelliforme enhanced desiccation tolerance in Nostoc sp. PCC 7120. Furthermore, a transcription factor Hrf1 (hlips-cluster repressor factor 1) was identified and shown to coordinately regulate the expression of hlips-cluster and desiccation-induced psbAs. Hrf1 acts as a negative regulator for the adaptation of N. flagelliforme to the harsh desert environment. Phylogenetic analysis revealed that most species in the Nostoc genus possess both tandemly repeated Hlips and Hrf1. Our results suggest convergent evolution of desiccation tolerance through the coevolution of tandem Hlips duplication and Hrf1 in subaerial Nostoc species, providing insights into the mechanism of desiccation tolerance in photosynthetic organisms.

Biliverdin incorporation into the cyanobacteriochrome SPI1085g3 from Spirulina

Cyanobacteriochromes (CBCRs) bind linear tetrapyrrole chromophores, mostly phycocyanobilin (PCB), and exhibit considerable spectral diversity with a high potential for biotechnological applications. Particular attention has been given to the conversion into intrinsic biliverdin (BV) incorporation due to the absence of PCB in mammalian cells. Our recent study discovered that a red/green CBCR of Spirulina subsalsa, SPI1085g3, was covalently attached to PCB and exhibited strong red fluorescence with a unique red/dark switch. In this study, we found that SPI1085g3 could be modestly chromophorylated with BV and absorb somewhat shifted (10 nm) red light, while the single C448S mutant could efficiently bind BV and exhibit unidirectional photoconversion and moderate dark reversion. The fluorescence in its dark-adapted state was switched off by red light, followed by a moderate recovery in the dark, and these were properties similar to those of PCB-binding SPI1085g3. Furthermore, by introducing the CY motif into the conserved CH motif for chromophore attachment, we developed another variant, C448S_CY, which showed increased BV-binding efficiency. As expected, C448S_CY had a significant enhancement in fluorescence quantum yield, reaching that of PCB-binding SPI1085g3 (0.14). These BV-binding CBCRs offer an improved platform for the development of unique photoswitchable fluorescent proteins compared with PCB-binding CBCRs.

Point (S-to-G) Mutations in the W(S/G)GE Motif in Red/Green Cyanobacteriochrome GAF Domains Enhance Thermal Reversion Rates

Cyanobacteriochromes (CBCRs) are photoreceptors consisting of single or tandem GAF (cGMP-phosphodiesterase/adenylate cyclase/FhlA) domains that bind bilin chromophores. Canonical red/green CBCR GAF domains are a well-characterized subgroup of the expanded red/green CBCR GAF domain family that binds phycocyanobilin (PCB) and converts between a thermally stable red-absorbing Pr state and a green-absorbing Pg state. The rate of thermal reversion from Pg to Pr varies widely among canonical red/green CBCR GAF domains, with half-lives ranging from days to seconds. Since the thermal reversion rate is an important parameter for the application of CBCR GAF domains as optogenetic tools, the molecular factors controlling the thermal reversion rate are of particular interest. Here, we report that point mutations in a well-conserved W(S/G)GE motif alter reversion rates in canonical red/green CBCR GAF domains in a predictable manner. Specifically, S-to-G mutations enhance thermal reversion rates, while the reverse, G-to-S mutations slow thermal reversion. Despite the distance (>10 Å) of the mutation site from the chromophore, molecular dynamics simulations and nuclear magnetic resonance (NMR) analyses suggest that the presence of a glycine residue allows the formation of a water bridge that alters the conformational dynamics of chromophore-interacting residues, leading to enhanced Pg to Pr thermal reversion.