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1. Cyanoexosortase B is essential for motility, biofilm formation, and scytonemin production in a filamentous cyanobacterium

   https://doi.org/10.1128/msphere.01006-24  

   Parrett, Gabriel A; Haft, Daniel H; Ruiz, Maida; Garcia-Pichel, Ferran; Ebmeier, Christopher C; Risser, Douglas D (June 2025, mSphere)

   Ellermeier, Craig D (Ed.)

   ABSTRACT Exosortases are involved in trafficking proteins containing PEP-CTERM domains to the exterior of gram-negative bacterial cells. The role of these proteins in cyanobacteria, where such homologs are common, has not been defined. The filamentous cyanobacteriumNostoc punctiformecontains a single putative exosortase, designated cyanoexosortase B (CrtB), implicated by previous work both in motility and in the production of the UV-absorbing pigment, scytonemin. To determine the role ofcrtBinN. punctiforme, acrtB-deletion strain (ΔcrtB) was generated. ΔcrtBpresented the loss of motility, biofilm formation, and scytonemin production. In the case of motility, the ΔcrtBmutant exhibited a specific defect in the ability of hormogonia (specialized motile filaments) to adhere to hormogonium polysaccharide (HPS), and several PEP-CTERM proteins expressed in motile hormogonia were differentially abundant in the exoproteome of the wild-type compared with the ΔcrtBstrain. These results are consistent with the hypothetical role of CrtB in the processing and export of PEP-CTERM proteins that play a critical role in stabilizing the interaction between the filament surface and HPS to facilitate motility and biofilm formation. In the case of scytonemin—the late biosynthetic steps of which occur in the periplasm and whose operon contains several putative PEP-CTERM proteins—ΔcrtBfailed to produce it. Given the abundance of putative PEP-CTERM proteins encoded in theN. punctiformegenome and the fact that this study only associates a fraction of them with biological functions, it seems likely that CrtB may play an important role in other biological processes in cyanobacteria.IMPORTANCEIn gram-negative bacteria, exosortases facilitate the trafficking of proteins to the exterior of the cell where they have been implicated in stabilizing the association of extracellular polymeric substances (EPS) with the cell surface to facilitate biofilm formation and flocculation, but the role of exosortases in cyanobacteria has not been explored. Here, we characterize the role of cyanoexosortase B (CrtB) in the filamentous cyanobacteriumNostoc punctiforme, demonstrating thatcrtBis essential for motility, biofilm formation, and the production of the sunscreen pigment scytonemin. These findings have important implications for understanding motility and biofilm formation in filamentous cyanobacteria as well as efforts toward the heterologous production of scytonemin in non-native hosts. 

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2. Photosynthetic modulation during the diurnal cycle in a unicellular diazotrophic cyanobacterium grown under nitrogen-replete and nitrogen-fixing conditions

   https://doi.org/10.1038/s41598-022-21829-6  

   Liberton, Michelle; Biswas, Sandeep; Pakrasi, Himadri B. (November 2022, Scientific Reports)

   Abstract Cyanobacteria are the only oxygenic photosynthetic organisms that can fix nitrogen. In diazotrophic cyanobacteria, the regulation of photosynthesis during the diurnal cycle is hypothesized to be linked with nitrogen fixation and involve the D1 protein isoform PsbA4. The amount of bioavailable nitrogen has a major impact on productivity in aqueous environments. In contrast to low- or nitrogen-fixing (−N) conditions, little data on photosynthetic regulation under nitrogen-replete (+ N) conditions are available. We compared the regulation of photosynthesis under −N and + N conditions during the diurnal cycle in wild type and apsbA4deletion strain of the unicellular diazotrophic cyanobacteriumCyanothecesp. ATCC 51142. We observed common changes to light harvesting and photosynthetic electron transport during the dark in + N and −N conditions and found that these modifications occur in both diazotrophic and non-diazotrophic cyanobacteria. Nitrogen availability increased PSII titer when cells transitioned from dark to light and promoted growth. Under −N conditions, deletion of PsbA4 modified charge recombination in dark and regulation of PSII titer during dark to light transition. We conclude that darkness impacts the acceptor-side modifications to PSII and photosynthetic electron transport in cyanobacteria independently of the nitrogen-fixing status and the presence of PsbA4. 

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3. A DnaK(Hsp70) Chaperone System Connects Type IV Pilus Activity to Polysaccharide Secretion in Cyanobacteria

   https://doi.org/10.1128/mbio.00514-22  

   McDonald, Heather J.; Kweon, HoJun; Kurnfuli, Shadi; Risser, Douglas D. (April 2022, mBio)

   Sogaard-Andersen, Lotte (Ed.)

   ABSTRACT Surface motility powered by type IV pili (T4P) is widespread among bacteria, including the photosynthetic cyanobacteria. This form of movement typically requires the deposition of a motility-associated polysaccharide, and several studies indicate that there is complex coregulation of T4P motor activity and polysaccharide production, although a mechanistic understanding of this coregulation is not fully defined. Here, using a combination of genetic, comparative genomic, transcriptomic, protein-protein interaction, and cytological approaches in the model filamentous cyanobacterium N. punctiforme , we provided evidence that a DnaK-type chaperone system coupled the activity of the T4P motors to the production of the motility-associated hormogonium polysaccharide (HPS). The results from these studies indicated that DnaK1 and DnaJ3 along with GrpE comprised a chaperone system that interacted specifically with active T4P motors and was required to produce HPS. Genomic conservation in cyanobacteria and the conservation of the protein-protein interaction network in the model unicellular cyanobacterium Synechocystis sp. strain PCC 6803 imply that this system is conserved among nearly all motile cyanobacteria and provides a mechanism to coordinate polysaccharide secretion and T4P activity in these organisms. IMPORTANCE Many bacteria, including photosynthetic cyanobacteria, exhibit type IV pili (T4P) driven surface motility. In cyanobacteria, this form of motility facilitates dispersal, phototaxis, the formation of supracellular structures, and the establishment of nitrogen-fixing symbioses with eukaryotes. T4P-powered motility typically requires the deposition of motility-associated polysaccharides, and previous studies indicate that T4P activity and polysaccharide production are intimately linked. However, the mechanism by which these processes are coupled is not well defined. Here, we identified and characterized a DnaK(Hsp70)-type chaperone system that coordinates these two processes in cyanobacteria. 

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4. Phototaxis in a wild isolate of the cyanobacterium Synechococcus elongatus

   https://doi.org/10.1073/pnas.1812871115  

   Yang, Yiling; Lam, Vinson; Adomako, Marie; Simkovsky, Ryan; Jakob, Annik; Rockwell, Nathan C.; Cohen, Susan E.; Taton, Arnaud; Wang, Jingtong; Lagarias, J. Clark; et al (December 2018, Proceedings of the National Academy of Sciences)

   Many cyanobacteria, which use light as an energy source via photosynthesis, have evolved the ability to guide their movement toward or away from a light source. This process, termed “phototaxis,” enables organisms to localize in optimal light environments for improved growth and fitness. Mechanisms of phototaxis have been studied in the coccoid cyanobacteriumSynechocystissp. strain PCC 6803, but the rod-shapedSynechococcus elongatusPCC 7942, studied for circadian rhythms and metabolic engineering, has no phototactic motility. In this study we report a recent environmental isolate ofS. elongatus, the strain UTEX 3055, whose genome is 98.5% identical to that of PCC 7942 but which is motile and phototactic. A six-gene operon encoding chemotaxis-like proteins was confirmed to be involved in phototaxis. Environmental light signals are perceived by a cyanobacteriochrome, PixJ_Se(Synpcc7942_0858), which carries five GAF domains that are responsive to blue/green light and resemble those of PixJ fromSynechocystis. Plate-based phototaxis assays indicate that UTEX 3055 uses PixJ_Seto sense blue and green light. Mutation of conserved functional cysteine residues in different GAF domains indicates that PixJ_Secontrols both positive and negative phototaxis, in contrast to the multiple proteins that are employed for implementing bidirectional phototaxis inSynechocystis. 

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5. The McdAB system positions α‐carboxysomes in proteobacteria

   https://doi.org/10.1111/mmi.14708  

   MacCready, Joshua S.; Tran, Lisa; Basalla, Joseph L.; Hakim, Pusparanee; Vecchiarelli, Anthony G. (March 2021, Molecular Microbiology)

   Abstract Carboxysomes are protein‐based organelles essential for carbon fixation in cyanobacteria and proteobacteria. Previously, we showed that the cyanobacterial nucleoid is used to equally space out β‐carboxysomes across cell lengths by a two‐component system (McdAB) in the model cyanobacteriumSynechococcus elongatusPCC 7942. More recently, we found that McdAB systems are widespread among β‐cyanobacteria, which possess β‐carboxysomes, but are absent in α‐cyanobacteria, which possess structurally and phyletically distinct α‐carboxysomes. Cyanobacterial α‐carboxysomes are thought to have arisen in proteobacteria and then horizontally transferred into cyanobacteria, which suggests that α‐carboxysomes in proteobacteria may also lack the McdAB system. Here, using the model chemoautotrophic proteobacteriumHalothiobacillus neapolitanus, we show that a McdAB system distinct from that of β‐cyanobacteria operates to position α‐carboxysomes across cell lengths. We further show that this system is widespread among α‐carboxysome‐containing proteobacteria and that cyanobacteria likely inherited an α‐carboxysome operon from a proteobacterium lacking themcdABlocus. These results demonstrate that McdAB is a cross‐phylum two‐component system necessary for positioning both α‐ and β‐carboxysomes. The findings have further implications for understanding the positioning of other protein‐based bacterial organelles involved in diverse metabolic processes. Plain language summaryCyanobacteria are well known to fix atmospheric CO₂into sugars using the enzyme Rubisco. Less appreciated are the carbon‐fixing abilities of proteobacteria with diverse metabolisms. Bacterial Rubisco is housed within organelles called carboxysomes that increase enzymatic efficiency. Here we show that proteobacterial carboxysomes are distributed in the cell by two proteins, McdA and McdB. McdA on the nucleoid interacts with McdB on carboxysomes to equidistantly space carboxysomes from one another, ensuring metabolic homeostasis and a proper inheritance of carboxysomes following cell division. This study illuminates how widespread carboxysome positioning systems are among diverse bacteria. Carboxysomes significantly contribute to global carbon fixation; therefore, understanding the spatial organization mechanism shared across the bacterial world is of great interest. 

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