The role of HetN in maintenance of the heterocyst pattern in Anabaena sp. PCC 7120

Characterizing the Impact of Communication on Cellular and Collective Behavior Using a Three-Dimensional Multiscale Cellular Model

Communication between cells enables the coordination that drives structural and functional complexity in biological systems. Both single and multicellular organisms have evolved diverse communication systems for a range of purposes, including synchronization of behavior, division of labor, and spatial organization. Synthetic systems are also increasingly being engineered to utilize cell-cell communication. While research has elucidated the form and function of cell-cell communication in many biological systems, our knowledge is still limited by the confounding effects of other biological phenomena at play and the bias of the evolutionary background. In this work, our goal is to push forward the context-free understanding of what impact cell-cell communication can have on cellular and population behavior to more fully understand the extent to which cell-cell communication systems can be utilized, modified, and engineered. We use an in silico model of 3D multiscale cellular populations, with dynamic intracellular networks interacting via diffusible signals. We focus on two key communication parameters: the effective interaction distance at which cells are able to interact and the receptor activation threshold. We found that cell-cell communication can be divided into six different forms along the parameter axes, three asocial and three social. We also show that cellular behavior, tissue composition, and tissue diversity are all highly sensitive to both the general form and specific parameters of communication even when the cellular network has not been biased towards that behavior.

Mathematical models of nitrogen-fixing cell patterns in filamentous cyanobacteria

The Anabaena genus is a model organism of filamentous cyanobacteria whose vegetative cells can differentiate under nitrogen-limited conditions into a type of cell called a heterocyst. These heterocysts lose the possibility to divide and are necessary for the filament because they can fix and share environmental nitrogen. In order to distribute the nitrogen efficiently, heterocysts are arranged to form a quasi-regular pattern whose features are maintained as the filament grows. Recent efforts have allowed advances in the understanding of the interactions and genetic mechanisms underlying this dynamic pattern. Here, we present a systematic review of the existing theoretical models of nitrogen-fixing cell differentiation in filamentous cyanobacteria. These filaments constitute one of the simplest forms of multicellular organization, and this allows for several modeling scales of this emergent pattern. The system has been approached at three different levels. From bigger to smaller scale, the system has been considered as follows: at the population level, by defining a mean-field simplified system to study the ratio of heterocysts and vegetative cells; at the filament level, with a continuous simplification as a reaction-diffusion system; and at the cellular level, by studying the genetic regulation that produces the patterning for each cell. In this review, we compare these different approaches noting both the virtues and shortcomings of each one of them.

A proteolytic pathway coordinates cell division and heterocyst differentiation in the cyanobacterium Anabaena sp. PCC 7120

The filamentous, multicellular cyanobacterium Anabaena sp. PCC 7120 (Anabaena) is a prokaryotic model for the study of cell differentiation and cell-cell interactions. Upon combined-nitrogen deprivation, Anabaena forms a particular cell type, heterocyst, for aerobic nitrogen fixation. Heterocysts are semiregularly spaced among vegetative cells. Heterocyst differentiation is coupled to cell division, but the underlying mechanism remains unclear. This mechanism could be mediated by the putative protease HetF, which is a divisome component and is necessary for heterocyst differentiation. In this study, by suppressor screening, we identified PatU3, as a negative regulator acting downstream of HetF for cell division and heterocyst development. The inactivation of patU3 restored the capacity of cell division and heterocyst differentiation in the ΔhetF mutant, and overexpression of patU3 inhibited both processes in the wild-type background. We demonstrated that PatU3 was a specific substrate of the protease activity of HetF. Consequently, PatU3 accumulated in the hetF-deficient mutant, which was responsible for the resultant mutant phenotype. The cleavage site of PatU3 by HetF was mapped after the Arg117 residue, whose mutation made PatU3 resistant to HetF processing, and mimicked the effect of hetF deletion. Our results provided evidence that HetF regulated cell division and heterocyst differentiation by controlling the inhibitory effects of PatU3. This proteolytic pathway constituted a mechanism for the coordination between cell division and differentiation in a prokaryotic model used for studies on developmental biology and multicellularity.

Terminal heterocyst differentiation in the Anabaena patA mutant as a result of post-transcriptional modifications and molecular leakage

The Anabaena genus is a model organism of filamentous cyanobacteria whose vegetative cells can differentiate under nitrogen-limited conditions into a type of cell called heterocyst. These heterocysts lose the possibility to divide and are necessary for the colony because they can fix and share environmental nitrogen. In order to distribute the nitrogen efficiently, heterocysts are arranged to form a quasi-regular pattern whose features are maintained as the filament grows. Recent efforts have allowed advances in the understanding of the interactions and genetic mechanisms underlying this dynamic pattern. However, the main role of the patA and hetF genes are yet to be clarified; in particular, the patA mutant forms heterocysts almost exclusively in the terminal cells of the filament. In this work, we investigate the function of these genes and provide a theoretical model that explains how they interact within the broader genetic network, reproducing their knock-out phenotypes in several genetic backgrounds, including a nearly uniform concentration of HetR along the filament for the patA mutant. Our results suggest a role of hetF and patA in a post-transcriptional modification of HetR which is essential for its regulatory function. In addition, the existence of molecular leakage out of the filament in its boundary cells is enough to explain the preferential appearance of terminal heterocysts, without any need for a distinct regulatory pathway.

Understanding multicellular pattern formation is key for the study of both natural and synthetic developmental processes. Arguably one of the simplest model systems for this is the filamentous cyanobacterium Anabaena, that in conditions of nitrogen deprivation undergoes a dynamical differentiation process that differentiates roughly one in every ten cells into nitrogen-fixing heterocysts, in a quasi-regular pattern that is maintained as the filament keeps growing. One of the most characteristic mutations affecting this process forms heterocysts mostly constrained to the terminal cells of the filament. We have used experimental observations to propose a mathematical model of heterocyst differentiation able to reproduce this striking phenotype. The model extends our understanding of the regulations in this pattern-forming system and makes several predictions on molecular interactions. Importantly, a key aspect is the boundary condition at the filament’s ends: inhibitors of differentiation should be able to leak out of the filament, or otherwise the terminal cells would not differentiate. This highlights, in a very clear example, the importance of considering physical constraints in developmental processes.

The Making of a Heterocyst in Cyanobacteria

Heterocyst differentiation that occurs in some filamentous cyanobacteria, such as Anabaena sp. PCC 7120, provides a unique model for prokaryotic developmental biology. Heterocyst cells are formed in response to combined-nitrogen deprivation and possess a microoxic environment suitable for nitrogen fixation following extensive morphological and physiological reorganization. A filament of Anabaena is a true multicellular organism, as nitrogen and carbon sources are exchanged among different cells and cell types through septal junctions to ensure filament growth. Because heterocysts are terminally differentiated cells and unable to divide, their activity is an altruistic behavior dedicated to providing fixed nitrogen for neighboring vegetative cells. Heterocyst development is also a process of one-dimensional pattern formation, as heterocysts are semiregularly intercalated among vegetative cells. Morphogens form gradients along the filament and interact with each other in a fashion that fits well into the Turing model, a mathematical framework to explain biological pattern formation. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

c-di-GMP Homeostasis Is Critical for Heterocyst Development in Anabaena sp. PCC 7120

c-di-GMP is a ubiquitous bacterial signal regulating various physiological process. Anabaena PCC 7120 (Anabaena) is a filamentous cyanobacterium able to form regularly-spaced heterocysts for nitrogen fixation, in response to combined-nitrogen deprivation in 24h. Anabaena possesses 16 genes encoding proteins for c-di-GMP metabolism, and their functions are poorly characterized, except all2874 (cdgS) whose deletion causes a decrease in heterocyst frequency 48h after nitrogen starvation. We demonstrated here that c-di-GMP levels increased significantly in Anabaena after combined-nitrogen starvation. By inactivating each of the 16 genes, we found that the deletion of all1175 (cdgSH) led to an increase of heterocyst frequency 24h after nitrogen stepdown. A double mutant ΔcdgSHΔcdgS had an additive effect over the single mutants in regulating heterocyst frequency, indicating that the two genes acted at different time points for heterocyst spacing. Biochemical and genetic data further showed that the functions of CdgSH and CdgS in the setup or maintenance of heterocyst frequency depended on their opposing effects on the intracellular levels of c-di-GMP. Finally, we demonstrated that heterocyst differentiation was completely inhibited when c-di-GMP levels became too high or too low. Together, these results indicate that the homeostasis of c-di-GMP level is important for heterocyst differentiation in Anabaena.

The developmental regulator PatD modulates assembly of the cell-division protein FtsZ in the cyanobacterium Anabaena sp. PCC 7120

FtsZ is a tubulin-like GTPase that polymerizes to initiate the process of cell division in bacteria. Heterocysts are terminally differentiated cells of filamentous cyanobacteria that have lost the capacity for cell division and in which the ftsZ gene is downregulated. However, mechanisms of FtsZ regulation during heterocyst differentiation have been scarcely investigated. The patD gene is NtcA dependent and involved in the optimization of heterocyst frequency in Anabaena sp. PCC 7120. Here, we report that the inactivation of patD caused the formation of multiple FtsZ-rings in vegetative cells, cell enlargement, and the retention of peptidoglycan synthesis activity in heterocysts, whereas its ectopic expression resulted in aberrant FtsZ polymerization and cell division. PatD interacted with FtsZ, increased FtsZ precipitation in sedimentation assays, and promoted the formation of thick straight FtsZ bundles that differ from the toroidal aggregates formed by FtsZ alone. These results suggest that in the differentiating heterocysts, PatD interferes with the assembly of FtsZ. We propose that in Anabaena FtsZ is a bifunctional protein involved in both vegetative cell division and regulation of heterocyst differentiation. In the differentiating cells PatD-FtsZ interactions appear to set an FtsZ activity that is insufficient for cell division but optimal to foster differentiation.

The Dual Role of the Glycolipid Envelope in Different Cell Types of the Multicellular Cyanobacterium Anabaena variabilis ATCC 29413

Anabaena variabilis is a filamentous cyanobacterium that is capable to differentiate specialized cells, the heterocysts and akinetes, to survive under different stress conditions. Under nitrogen limited condition, heterocysts provide the filament with nitrogen by fixing N₂. Akinetes are spore-like dormant cells that allow survival during adverse environmental conditions. Both cell types are characterized by the presence of a thick multilayered envelope, including a glycolipid layer. While in the heterocyst this glycolipid layer is required for the maintenance of a microoxic environment and nitrogen fixation, its function in akinetes is completely unknown. Therefore, we constructed a mutant deficient in glycolipid synthesis and investigated the performance of heterocysts and akinetes in that mutant strain. We chose to delete the gene Ava_2595, which is homolog to the known hglB gene, encoding a putative polyketide synthase previously shown to be involved in heterocyst glycolipid synthesis in Anabaena sp. PCC 7120, a species which does not form akinetes. Under the respective conditions, the Ava_2595 null mutant strain formed aberrant heterocysts and akinete-like cells, in which the specific glycolipid layers were absent. This confirmed firstly that both cell types use a glycolipid of identical chemical composition in their special envelopes and, secondly, that HglB is essential for glycolipid synthesis in both types of differentiated cells. As a consequence, the mutant was not able to fix N₂ and to grow under diazotrophic conditions. Furthermore, the akinetes lacking the glycolipids showed a severely reduced tolerance to stress conditions, but could germinate normally under standard conditions. This demonstrates the importance of the glycolipid layer for the ability of akinetes as spore-like dormant cells to withstand freezing, desiccation, oxidative stress and attack by lytic enzymes. Our study established the dual role of the glycolipid layer in fulfilling different functions in the evolutionary-related specialized cells of cyanobacteria. It also indicates the existence of a common pathway involving HglB for the synthesis of glycolipids in heterocysts and akinetes.

Inactivation of Three RG(S/T)GR Pentapeptide-Containing Negative Regulators of HetR Results in Lethal Differentiation of Anabaena PCC 7120

The filamentous cyanobacterium Anabaena sp. PCC 7120 produces, during the differentiation of heterocysts, a short peptide PatS and a protein HetN, both containing an RGSGR pentapeptide essential for activity. Both act on the master regulator HetR to guide heterocyst pattern formation by controlling the binding of HetR to DNA and its turnover. A third small protein, PatX, with an RG(S/T)GR motif is present in all HetR-containing cyanobacteria. In a nitrogen-depleted medium, inactivation of patX does not produce a discernible change in phenotype, but its overexpression blocks heterocyst formation. Mutational analysis revealed that PatX is not required for normal intercellular signaling, but it nonetheless is required when PatS is absent to prevent rapid ectopic differentiation. Deprivation of all three negative regulators—PatS, PatX, and HetN—resulted in synchronous differentiation. However, in a nitrogen-containing medium, such deprivation leads to extensive fragmentation, cell lysis, and aberrant differentiation, while either PatX or PatS as the sole HetR regulator can establish and maintain a semiregular heterocyst pattern. These results suggest that tight control over HetR by PatS and PatX is needed to sustain vegetative growth and regulated development. The mutational analysis has been interpreted in light of the opposing roles of negative regulators of HetR and the positive regulator HetL.

Expression from DIF1-motif promoters of hetR and patS is dependent on HetZ and modulated by PatU3 during heterocyst differentiation

HetR and PatS/PatX-derived peptides are the activator and diffusible inhibitor for cell differentiation and patterning in heterocyst-forming cyanobacteria. HetR regulates target genes via HetR-recognition sites. However, some genes (such as patS/patX) upregulated at the early stage of heterocyst differentiation possess DIF1 (or DIF⁺) motif (TCCGGA) promoters rather than HetR-recognition sites; hetR possesses both predicted regulatory elements. How HetR controls heterocyst-specific expression from DIF1 motif promoters remains to be answered. This study presents evidence that the expression from DIF1 motif promoters of hetR, patS and patX is more directly dependent on hetZ, a gene regulated by HetR via a HetR-recognition site. The HetR-binding site upstream of hetR is not required for the autoregulation of hetR. PatU3 (3′ portion of PatU) that interacts with HetZ may modulate the expression of hetR, hetZ and patS. These findings contribute to understanding of the mutual regulation of hetR, hetZ-patU and patS/patX in a large group of multicellular cyanobacteria.

hetN and patS Mutations Enhance Accumulation of Fatty Alcohols in the hglT Mutants of Anabaena sp. PCC 7120

The heterocysts present in filamentous cyanobacteria such as Anabaena sp. PCC 7120 are known to be regulated by HetN and PatS, the repressors of heterocyst differentiation; therefore, the inactivation of these proteins will result in the formation of multiple heterocysts. To enhance the accumulation of fatty alcohols synthesized in the heterocyst, we introduced mutations of these repressors to increase heterocyst frequency. First, we isolated double mutants of hetN and patS and confirmed that the null mutation of these genes promoted higher frequencies of heterocyst formation and higher accumulation of heterocyst-specific glycolipids (Hgls) compared with its wild type. Next, we combined hetN and patS mutations with an hglT (encoding glycosyltransferase, an enzyme involved in Hgl synthesis) mutation to increase the accumulation of fatty alcohols since knockout mutation of hglT results in accumulation of very long chain fatty alcohol, the precursor of Hgl. We also observed retarded growth, lower chlorophyll content and up to a five-fold decrease in photosynthetic activity of the hetN/patS/hglT triple mutants. In contrast, the triple mutants showed three times higher heterocyst formation frequencies than the hglT single mutant and wild type. The production rate of fatty alcohol in the triple mutants attained a value 1.41 nmol/mL OD₇₃₀, whereas accumulation of Hgls in the wild type was 0.90 nmol/mL OD₇₃₀. Aeration of culture improved the accumulation of fatty alcohols in hetN/patS/hglT mutant cells up to 2.97 nmol/mL OD₇₃₀ compared with cells cultured by rotation. Our study outlines an alternative strategy for fatty alcohol production supported by photosynthesis and nitrogen fixation.

patD, a Gene Regulated by NtcA, Is Involved in the Optimization of Heterocyst Frequency in the Cyanobacterium Anabaena sp. Strain PCC 7120

In the filamentous multicellular cyanobacterium Anabaena sp. strain PCC 7120, 5 to 10% of the cells differentiate into heterocysts, which are specialized in N₂ fixation. Heterocysts and vegetative cells are mutually dependent for filament growth through nutrient exchange. Thus, the heterocyst frequency should be optimized to maintain the cellular carbon and nitrogen (C/N) balance for filament fitness in the environment. Here, we report the identification of patD, whose expression is directly activated in developing cells by the transcription factor NtcA. The inactivation of patD increases heterocyst frequency and promotes the upregulation of the positive regulator of heterocyst development hetR, whereas its overexpression decreases the heterocyst frequency. The change in heterocyst frequency resulting from the inactivation of patD leads to the reduction in competitiveness of the filaments under combined-nitrogen-depleted conditions. These results indicate that patD regulates heterocyst frequency in Anabaena sp. PCC 7120, ensuring its optimal filament growth.IMPORTANCE Microorganisms have evolved various strategies in order to adapt to the environment and compete with other organisms. Heterocyst differentiation is a prokaryotic model for studying complex cellular regulation. The NtcA-regulated gene patD controls the ratio of heterocysts relative to vegetative cells on the filaments of Anabaena sp. strain PCC 7120. Such a regulation provides a mechanism through which carbon fixation by vegetative cells and nitrogen fixation by heterocysts are properly balanced to ensure optimal growth and keep a competitive edge for long-term survival.

New Applications of Synthetic Biology Tools for Cyanobacterial Metabolic Engineering

Cyanobacteria are promising microorganisms for sustainable biotechnologies, yet unlocking their potential requires radical re-engineering and application of cutting-edge synthetic biology techniques. In recent years, the available devices and strategies for modifying cyanobacteria have been increasing, including advances in the design of genetic promoters, ribosome binding sites, riboswitches, reporter proteins, modular vector systems, and markerless selection systems. Because of these new toolkits, cyanobacteria have been successfully engineered to express heterologous pathways for the production of a wide variety of valuable compounds. Cyanobacterial strains with the potential to be used in real-world applications will require the refinement of genetic circuits used to express the heterologous pathways and development of accurate models that predict how these pathways can be best integrated into the larger cellular metabolic network. Herein, we review advances that have been made to translate synthetic biology tools into cyanobacterial model organisms and summarize experimental and in silico strategies that have been employed to increase their bioproduction potential. Despite the advances in synthetic biology and metabolic engineering during the last years, it is clear that still further improvements are required if cyanobacteria are to be competitive with heterotrophic microorganisms for the bioproduction of added-value compounds.

Expanding the Potential of CRISPR-Cpf1-Based Genome Editing Technology in the Cyanobacterium Anabaena PCC 7120

CRISPR systems, such as CRISPR-Cas9 and CRISPR-Cpf1, have been successfully used for genome editing in a variety of organisms. Although the technique of CRISPR-Cpf1 has been applied in cyanobacteria recently, its use was limited without exploiting the full potential of such a powerful genetic system. Using the cyanobacterium Anabaena PCC 7120 as a model strain, we improved the tools and designed genetic strategies based on CRISPR-Cpf1, which enabled us to realize genetic experiments that have been so far difficult to do in cyanobacteria. The development includes: (1) a "two-spacers" strategy for single genomic modification, with a success rate close to 100%; (2) rapid multiple genome editing using editing plasmids with different resistance markers; (3) using sacB, a counter-selection marker conferring sucrose sensitivity, to enable the active loss of the editing plasmids and facilitate multiple rounds of genetic modification or phenotypic analysis; (4) manipulation of essential genes by the creation of conditional mutants, using as example, polA encoding the DNA polymerase I essential for DNA replication and repair; (5) large DNA fragment deletion, up to 118 kb, from the Anabaena chromosome, corresponding to the largest bacterial chromosomal region removed with CRISPR systems so far. The genome editing vectors and the strategies developed here will expand our ability to study and engineer cyanobacteria, which are extensively used for fundamental studies, biotechnological applications including biofuel production, and synthetic biology research. The vectors developed here have a broad host range, and could be readily used for genetic modification in other microorganisms.

Morphological and isotopic changes of heterocystous cyanobacteria in response to N2 partial pressure

Earth's atmospheric composition has changed significantly over geologic time. Many redox active atmospheric constituents have left evidence of their presence, while inert constituents such as dinitrogen gas (N₂ ) are more elusive. In this study, we examine two potential biological indicators of atmospheric N₂ : the morphological and isotopic signatures of heterocystous cyanobacteria. Biological nitrogen fixation constitutes the primary source of fixed nitrogen to the global biosphere and is catalyzed by the oxygen-sensitive enzyme nitrogenase. To protect this enzyme, some filamentous cyanobacteria restrict nitrogen fixation to microoxic cells (heterocysts) while carrying out oxygenic photosynthesis in vegetative cells. Heterocysts terminally differentiate in a pattern that is maintained as the filaments grow, and nitrogen fixation imparts a measurable isotope effect, creating two biosignatures that have previously been interrogated under modern N₂ partial pressure (pN₂ ) conditions. Here, we examine the effect of variable pN₂ on these biosignatures for two species of the filamentous cyanobacterium Anabaena. We provide the first in vivo estimate of the intrinsic isotope fractionation factor of Mo-nitrogenase (ε_fix  = -2.71 ± 0.09‰) and show that, with decreasing pN₂ , the net nitrogen isotope fractionation decreases for both species, while the heterocyst spacing decreases for Anabaena cylindrica and remains unchanged for Anabaena variabilis. These results are consistent with the nitrogen fixation mechanisms available in the two species. Application of these quantifiable effects to the geologic record may lead to new paleobarometric measurements for pN₂ , ultimately contributing to a better understanding of Earth's atmospheric evolution.

Cyanobacterial Septal Junctions: Properties and Regulation

Heterocyst-forming cyanobacteria are multicellular organisms that grow as chains of cells (filaments or trichomes) in which the cells exchange regulators and nutrients. In this article, we review the morphological, physiological and genetic data that have led to our current understanding of intercellular communication in these organisms. Intercellular molecular exchange appears to take place by simple diffusion through proteinaceous structures, known as septal junctions, which connect the adjacent cells in the filament and traverse the septal peptidoglycan through perforations known as nanopores. Proteins that are necessary to produce, and that may be components of, the septal junctions-SepJ, FraC and FraD-have been identified in the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 model. Additionally, several proteins that are necessary to produce a normal number of nanopores and functional septal junctions have been identified, including AmiC-type amidases, peptidoglycan-binding proteins and some membrane transporters. Available reports and reevaluation of intercellular molecular transfer data for some mutants of Anabaena suggest that the septal junctions can be regulated, likely by a mechanism of gating.

Ancient association of cyanobacterial multicellularity with the regulator HetR and an RGSGR pentapeptide-containing protein (PatX)

One simple model to explain biological pattern postulates the existence of a stationary regulator of differentiation that positively affects its own expression, coupled with a diffusible suppressor of differentiation that inhibits the regulator's expression. The first has been identified in the filamentous, heterocyst-forming cyanobacterium, Anabaena PCC 7120 as the transcriptional regulator, HetR and the second as the small protein, PatS, which contains a critical RGSGR motif that binds to HetR. HetR is present in almost all filamentous cyanobacteria, but only a subset of heterocyst-forming strains carry proteins similar to PatS. We identified a third protein, PatX that also carries the RGSGR motif and is coextensive with HetR. Amino acid sequences of PatX contain two conserved regions: the RGSGR motif and a hydrophobic N-terminus. Within 69 nt upstream from all instances of the gene is a DIF1 motif correlated in Anabaena with promoter induction in developing heterocysts, preceded in heterocyst-forming strains by an apparent NtcA-binding site, associated with regulation by nitrogen-status. Consistent with a role in the simple model, PatX is expressed dependent on HetR and acts to inhibit differentiation. The acquisition of the PatX/HetR pair preceded the appearance of both PatS and heterocysts, dating back to the beginnings of multicellularity.

Specific mutations in the permease domain of septal protein SepJ differentially affect functions related to multicellularity in the filamentous cyanobacterium Anabaena

Filamentous, heterocyst-forming cyanobacteria are multicellular organisms in which growth requires the activity of two interdependent cell types that exchange nutrients and regulators. Vegetative cells provide heterocysts with reduced carbon, and heterocysts provide vegetative cells with fixed nitrogen. Additionally, heterocyst differentiation from vegetative cells is regulated by inhibitors of differentiation produced by prospective heterocysts and heterocysts. Proteinaceous structures known as septal junctions join the cells in the filament. The SepJ protein is involved in formation of septal junctions in the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. SepJ bears extra-membrane and membrane (permease) domains and is located at the cell poles in the intercellular septa of the filament. Here we created Anabaena mutants that produce SepJ proteins altered in the permease domain. Some of these mutant SepJ proteins did not provide functions needed for Anabaena to form long filaments and (in some cases) differentiate heterocysts, identifying amino acids and amino acid stretches that are important for the structure or function of the protein. Some other mutant SepJ proteins fulfilled filamentation and heterocyst differentiation functions but failed to provide normal communication function assessed via the intercellular transfer of the fluorescent marker calcein. These mutant SepJ proteins bore mutations in amino acids located at the cytoplasmic face of the permease, which could affect access of the fluorescent marker to the septal junctions. Overall, the data are consistent with the idea that SepJ carries out multiple roles in the multicellular function of the Anabaena filament.

Genetic responses to carbon and nitrogen availability in Anabaena

Heterocyst-forming cyanobacteria are filamentous organisms that perform oxygenic photosynthesis and CO₂ fixation in vegetative cells and nitrogen fixation in heterocysts, which are formed under deprivation of combined nitrogen. These organisms can acclimate to use different sources of nitrogen and respond to different levels of CO₂ . Following work mainly done with the best studied heterocyst-forming cyanobacterium, Anabaena, here we summarize the mechanisms of assimilation of ammonium, nitrate, urea and N₂ , the latter involving heterocyst differentiation, and describe aspects of CO₂ assimilation that involves a carbon concentration mechanism. These processes are subjected to regulation establishing a hierarchy in the assimilation of nitrogen sources -with preference for the most reduced nitrogen forms- and a dependence on sufficient carbon. This regulation largely takes place at the level of gene expression and is exerted by a variety of transcription factors, including global and pathway-specific transcriptional regulators. NtcA is a CRP-family protein that adjusts global gene expression in response to the C-to-N balance in the cells, and PacR is a LysR-family transcriptional regulator (LTTR) that extensively acclimates the cells to oxygenic phototrophy. A cyanobacterial-specific transcription factor, HetR, is involved in heterocyst differentiation, and other LTTR factors are specifically involved in nitrate and CO₂ assimilation.

Mechanisms for Protein Redistribution in Thylakoids of Anabaena During Cell Differentiation

Thylakoid membranes are far from being homogeneous in composition. On the contrary, compositional heterogeneity of lipid and protein content is well known to exist in these membranes. The mechanisms for the confinement of proteins at a particular membrane domain have started to be unveiled, but we are far from a thorough understanding, and many issues remain to be elucidated. During the differentiation of heterocysts in filamentous cyanobacteria of the Anabaena and Nostoc genera, thylakoids undergo a complete reorganization, separating into two membrane domains of different appearance and subcellular localization. Evidence also indicates different functionality and protein composition for these two membrane domains. In this work, we have addressed the mechanisms that govern the specific localization of proteins at a particular membrane domain. Two classes of proteins were distinguished according to their distribution in the thylakoids. Our results indicate that the specific accumulation of proteins of the CURVATURE THYLAKOID 1 (CURT1) family and proteins containing the homologous CAAD domain at subpolar honeycomb thylakoids is mediated by multiple mechanisms including a previously unnoticed phenomenon of thylakoid membrane migration.

The Ser/Thr Kinase PknH Is Essential for Maintaining Heterocyst Pattern in the Cyanobacterium Anabaena sp. Strain PCC 7120

In the filamentous cyanobacterium Anabaena sp. strain, PCC 7120, heterocysts (which are nitrogen-fixing cells) are formed in the absence of combined nitrogen in the medium. Heterocysts are separated from one another by 10 to 15 vegetative cells along the filaments, which consist of a few hundred of cells. hetR is necessary for heterocyst differentiation; and patS and hetN, expressed in heterocysts, play important roles in heterocyst pattern formation by laterally inhibiting the expression of hetR in adjacent cells. The results of this study indicated that pknH, which encodes a Ser/Thr kinase, was also involved in heterocyst pattern formation. In the pknH mutant, the heterocyst pattern was normal within 24 h after nitrogen deprivation, but multiple contiguous heterocysts were formed from 24 to 48 h. A time-lapse analysis of reporter strains harboring a fusion between gfp and the hetR promoter indicated that pknH was required to suppress hetR expression in cells adjacent to the preexisting heterocysts. These results indicated that pknH was necessary for the lateral inhibition of heterocyst differentiation to maintain the heterocyst pattern.

Phenotypic Assessment Suggests Multiple Start Codons for HetN, an Inhibitor of Heterocyst Differentiation, in Anabaena sp. Strain PCC 7120

Multicellular organisms must carefully regulate the timing, number, and location of specialized cellular development. In the filamentous cyanobacterium Anabaena sp. strain PCC 7120, nitrogen-fixing heterocysts are interspersed between vegetative cells in a periodic pattern to achieve an optimal exchange of bioavailable nitrogen and reduced carbon. The spacing between heterocysts is regulated by the activity of two developmental inhibitors, PatS and HetN. PatS functions to create a de novo pattern from a homogenous field of undifferentiated cells, while HetN maintains the pattern throughout subsequent growth. Both PatS and HetN harbor the peptide motif ERGSGR, which is sufficient to inhibit development. While the small size of PatS makes the interpretation of inhibitory domains relatively simple, HetN is a 287-amino-acid protein with multiple functional regions. Previous work suggested the possibility of a truncated form of HetN containing the ERGSGR motif as the source of the HetN-derived inhibitory signal. In this work, we present evidence that the glutamate of the ERGSGR motif is required for proper HetN inhibition of heterocysts. Mutational analysis and subcellular localization indicate that the gene encoding HetN uses two methionine start codons (M1 and M119) to encode two protein forms: M1 is required for protein localization, while M119 is primarily responsible for inhibitory function. Finally, we demonstrate that patS and hetN are not functionally equivalent when expressed from the other gene's regulatory sequences. Taken together, these results help clarify the functional forms of HetN and will help refine future work defining a HetN-derived inhibitory signal in this model of one-dimensional periodic patterning.IMPORTANCE The proper placement of different cell types during a developmental program requires the creation and maintenance of a biological pattern to define the cells that will differentiate. Here we show that the HetN inhibitor, responsible for pattern maintenance of specialized nitrogen-fixing heterocyst cells in the filamentous cyanobacterium Anabaena, may be produced from two different start methionine codons. This work demonstrates that the two start sites are individually involved in a different HetN function, either membrane localization or inhibition of cellular differentiation.

The hetZ gene indirectly regulates heterocyst development at the level of pattern formation in Anabaena sp. strain PCC 7120

Multicellular development requires the careful orchestration of gene expression to correctly create and position specialized cells. In the filamentous cyanobacterium Anabaena sp. strain PCC 7120, nitrogen-fixing heterocysts are differentiated from vegetative cells in a reproducibly periodic and physiologically relevant pattern. While many genetic factors required for heterocyst development have been identified, the role of HetZ has remained unclear. Here, we present evidence to clarify the requirement of hetZ for heterocyst production and support a model where HetZ functions in the patterning stage of differentiation. We show that a clean, nonpolar deletion of hetZ fails to express the developmental genes hetR, patS, hetP and hetZ correctly and fails to produce heterocysts. Complementation and overexpression of hetZ in a hetP mutant revealed that hetZ was incapable of bypassing hetP, suggesting that it acts upstream of hetP. Complementation and overexpression of hetZ in a hetR mutant, however, demonstrated bypass of hetR, suggesting that it acts downstream of hetR and is capable of bypassing the need for hetR for differentiation irrespective of nitrogen status. Finally, protein-protein interactions were observed between HetZ and HetR, Alr2902 and HetZ itself. Collectively, this work suggests a regulatory role for HetZ in the patterning phase of cellular differentiation in Anabaena.

Functional Overlap of hetP and hetZ in Regulation of Heterocyst Differentiation in Anabaena sp. Strain PCC 7120

HetR plays a key role in regulation of heterocyst differentiation and patterning in Anabaena It directly regulates genes involved in heterocyst differentiation (such as hetP and hetZ), genes involved in pattern formation (patA), and many others. In this study, we investigated the functional relationship of hetP and hetZ and their role in HetR-dependent cell differentiation. Coexpression of hetP and hetZ from the promoter of ntcA, which encodes the global nitrogen regulator, enabled a hetR mutant to form heterocysts with low aerobic nitrogenase activity. Overexpression of hetZ restored heterocyst differentiation in a hetP mutant and vice versa. Overexpression of hetR restored heterocyst formation in either a hetP or a hetZ mutant but not in a hetZ hetP double mutant. The functional overlap of hetP and hetZ was further confirmed by reverse transcription-quantitative PCR (RT-qPCR) and transcriptomic analyses of their effects on gene expression. In addition, yeast two-hybrid and pulldown assays showed the interaction of HetZ with HetR. HetP and HetZ are proposed as the two major factors that control heterocyst formation in response to upregulation of hetRIMPORTANCE Heterocyst-forming cyanobacteria contribute significantly to N₂ fixation in marine, freshwater, and terrestrial ecosystems. Formation of heterocysts enables this group of cyanobacteria to fix N₂ efficiently under aerobic conditions. HetR, HetP, and HetZ are among the most important factors involved in heterocyst differentiation. We present evidence for the functional overlap of hetP and hetZ and for the key role of the HetR-HetP/HetZ circuit in regulation of heterocyst differentiation. The regulatory mechanism based on HetR, HetP, and HetZ is probably conserved in all heterocyst-forming cyanobacteria.

Role of PatS and cell type on the heterocyst spacing pattern in a filamentous branching cyanobacterium

Cell differentiation is one of the marks of multicellular organisms. Terminally specialised nitrogen-fixing cells, termed heterocysts, evolved in filamentous cyanobacteria more than 2 Gya. The development of their spacing pattern has been thoroughly investigated in model organisms such as Anabaena sp. PCC 7120. This paper focuses on the more complex, branching cyanobacterium Mastigocladus laminosus (Stigonematales). Contrary to what has been previously published, a heterocyst spacing pattern is present in M. laminosus but it varies with the age of the culture and the morphology of the cells. Heterocysts in young, narrow trichomes were more widely spaced (∼14.8 cells) than those in old, wide trichomes (∼9.4 cells). Biochemical and transgenic experiments reveal that the heterocyst spacing pattern is affected by the heterocyst inhibitor PatS. Addition of the pentapeptide RGSGR (PatS-5) to the growth medium and overexpression of patS from Anabaena sp. PCC 7120 in M. laminosus resulted in the loss of heterocyst differentiation under nitrogen deprivation. Bioinformatics investigations indicated that putative PatS sequences within cyanobacteria are highly diverse, and fall into two main clades. Both are present in most branching cyanobacteria. Despite its more complex, branching phenotype, M. laminosus appears to use a PatS-based pathway for heterocyst differentiation, a property shared by Anabaena/Nostoc.

Assessment of Anabaena sp. Strain PCC 7120 as a Heterologous Expression Host for Cyanobacterial Natural Products: Production of Lyngbyatoxin A

Cyanobacteria are well-known producers of natural products of highly varied structure and biological properties. However, the long doubling times, difficulty in establishing genetic methods for marine cyanobacteria, and low compound titers have hindered research into the biosynthesis of their secondary metabolites. While a few attempts to heterologously express cyanobacterial natural products have occurred, the results have been of varied success. Here, we report the first steps in developing the model freshwater cyanobacterium Anabaena sp. strain PCC 7120 (Anabaena 7120) as a general heterologous expression host for cyanobacterial secondary metabolites. We show that Anabaena 7120 can heterologously synthesize lyngbyatoxin A in yields comparable to those of the native producer, Moorea producens, and detail the design and use of replicative plasmids for compound production. We also demonstrate that Anabaena 7120 recognizes promoters from various biosynthetic gene clusters from both free-living and obligate symbiotic marine cyanobacteria. Through simple genetic manipulations, the titer of lyngbyatoxin A can be improved up to 13-fold. The development of Anabaena 7120 as a general heterologous expression host enables investigation of interesting cyanobacterial biosynthetic reactions and genetic engineering of their biosynthetic pathways.

Overexpression of SepJ alters septal morphology and heterocyst pattern regulated by diffusible signals in Anabaena

Filamentous, N2 -fixing, heterocyst-forming cyanobacteria grow as chains of cells that are connected by septal junctions. In the model organism Anabaena sp. strain PCC 7120, the septal protein SepJ is required for filament integrity, normal intercellular molecular exchange, heterocyst differentiation, and diazotrophic growth. An Anabaena strain overexpressing SepJ made wider septa between vegetative cells than the wild type, which correlated with a more spread location of SepJ in the septa as observed with a SepJ-GFP fusion, and contained an increased number of nanopores, the septal peptidoglycan perforations that likely accommodate septal junctions. The septa between heterocysts and vegetative cells, which are narrow in wild-type Anabaena, were notably enlarged in the SepJ-overexpressing mutant. Intercellular molecular exchange tested with fluorescent tracers was increased for the SepJ-overexpressing strain specifically in the case of calcein transfer between vegetative cells and heterocysts. These results support an association between calcein transfer, SepJ-related septal junctions, and septal peptidoglycan nanopores. Under nitrogen deprivation, the SepJ-overexpressing strain produced an increased number of contiguous heterocysts but a decreased percentage of total heterocysts. These effects were lost or altered in patS and hetN mutant backgrounds, supporting a role of SepJ in the intercellular transfer of regulatory signals for heterocyst differentiation.

Formation and maintenance of nitrogen-fixing cell patterns in filamentous cyanobacteria

Cyanobacteria forming one-dimensional filaments are paradigmatic model organisms of the transition between unicellular and multicellular living forms. Under nitrogen-limiting conditions, in filaments of the genus Anabaena, some cells differentiate into heterocysts, which lose the possibility to divide but are able to fix environmental nitrogen for the colony. These heterocysts form a quasiregular pattern in the filament, representing a prototype of patterning and morphogenesis in prokaryotes. Recent years have seen advances in the identification of the molecular mechanism regulating this pattern. We use these data to build a theory on heterocyst pattern formation, for which both genetic regulation and the effects of cell division and filament growth are key components. The theory is based on the interplay of three generic mechanisms: local autoactivation, early long-range inhibition, and late long-range inhibition. These mechanisms can be identified with the dynamics of hetR, patS, and hetN expression. Our theory reproduces quantitatively the experimental dynamics of pattern formation and maintenance for wild type and mutants. We find that hetN alone is not enough to play the role as the late inhibitory mechanism: a second mechanism, hypothetically the products of nitrogen fixation supplied by heterocysts, must also play a role in late long-range inhibition. The preponderance of even intervals between heterocysts arises naturally as a result of the interplay between the timescales of genetic regulation and cell division. We also find that a purely stochastic initiation of the pattern, without a two-stage process, is enough to reproduce experimental observations.

Mutation of the murC and murB Genes Impairs Heterocyst Differentiation in Anabaena sp. Strain PCC 7120

To stabilize cellular integrity in the face of environmental perturbations, most bacteria, including cyanobacteria, synthesize and maintain a strong, flexible, three-dimensional peptidoglycan lattice. Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium capable of differentiating morphologically distinct nitrogen-fixing heterocyst cells in a periodic pattern. While heterocyst development has been shown to require proper peptidoglycan remodeling, the role of peptidoglycan synthesis has remained unclear. Here we report the identification of two peptidoglycan synthesis genes, murC (alr5065) and murB (alr5066), as required for heterocyst development. The murC and murB genes are predicted to encode a UDP-N-acetylmuramate:L-alanine ligase and a UDP-N-acetylenolpyruvoylglucosamine reductase, respectively, and we confirm enzymatic function through complementation of Escherichia coli strains deficient for these enzymes. Cells depleted of either murC or murB expression failed to differentiate heterocysts under normally inducing conditions and displayed decreased filament integrity. To identify the stage(s) of development affected by murC or murB depletion, the spatial distribution of expression of the patterning marker gene, patS, was examined. Whereas murB depletion did not affect the pattern of patS expression, murC depletion led to aberrant expression of patS in all cells of the filament. Finally, expression of gfp controlled by the region of DNA immediately upstream of murC was enriched in differentiating cells and was repressed by the transcription factor NtcA. Collectively, the data in this work provide evidence for a direct link between peptidoglycan synthesis and the maintenance of a biological pattern in a multicellular organism.

IMPORTANCE

Multicellular organisms that differentiate specialized cells must regulate morphological changes such that both cellular integrity and the dissemination of developmental signals are preserved. Here we show that the multicellular bacterium Anabaena, which differentiates a periodic pattern of specialized heterocyst cells, requires peptidoglycan synthesis by the murine ligase genes murC (alr5065) and murB (alr5066) for maintenance of patterned gene expression, filament integrity, and overall development. This work highlights the significant influence that intracellular structure and intercellular connections can have on the execution of a developmental program.

Division of Labor in Biofilms: the Ecology of Cell Differentiation

The dense aggregation of cells on a surface, as seen in biofilms, inevitably results in both environmental and cellular heterogeneity. For example, nutrient gradients can trigger cells to differentiate into various phenotypic states. Not only do cells adapt physiologically to the local environmental conditions, but they also differentiate into cell types that interact with each other. This allows for task differentiation and, hence, the division of labor. In this article, we focus on cell differentiation and the division of labor in three bacterial species: Myxococcus xanthus, Bacillus subtilis, and Pseudomonas aeruginosa. During biofilm formation each of these species differentiates into distinct cell types, in some cases leading to cooperative interactions. The division of labor and the cooperative interactions between cell types are assumed to yield an emergent ecological benefit. Yet in most cases the ecological benefits have yet to be elucidated. A notable exception is M. xanthus, in which cell differentiation within fruiting bodies facilitates the dispersal of spores. We argue that the ecological benefits of the division of labor might best be understood when we consider the dynamic nature of both biofilm formation and degradation.

Efficient Gene Induction and Endogenous Gene Repression Systems for the Filamentous Cyanobacterium Anabaena sp. PCC 7120

In the last decade, many studies have been conducted to employ genetically engineered cyanobacteria in the production of various metabolites. However, the lack of a strict gene regulation system in cyanobacteria has hampered these attempts. The filamentous cyanobacterium Anabaena sp. PCC 7120 performs both nitrogen and carbon fixation and is, therefore, a good candidate organism for such production. To employ Anabaena cells for this purpose, we intended to develop artificial gene regulation systems to alter the cell metabolic pathways efficiently. We introduced into Anabaena a transcriptional repressor TetR, widely used in diverse organisms, and green fluorescent protein (GFP) as a reporter. We found that anhydrotetracycline (aTc) substantially induced GFP fluorescence in a concentration-dependent manner. By expressing tetR under the nitrate-specific promoter nirA, we successfully reduced the concentration of aTc required for the induction of gfp under nitrogen fixation conditions (to 10% of the concentration needed under nitrate-replete conditions). Further, we succeeded in the overexpression of GFP by depletion of nitrate without the inducer by means of promoter engineering of the nirA promoter. Moreover, we applied these gene regulation systems to a metabolic enzyme in Anabaena and successfully repressed glnA, the gene encoding glutamine synthetase that is essential for nitrogen assimilation in cyanobacteria, by expressing the small antisense RNA for glnA. Consequently, the ammonium production of an ammonium-excreting Anabaena mutant was significantly enhanced. We therefore conclude that the gene regulation systems developed in this study are useful tools for the regulation of metabolic enzymes and will help to increase the production of desired substances in Anabaena.

ABC Transporter Required for Intercellular Transfer of Developmental Signals in a Heterocystous Cyanobacterium

In the filamentous cyanobacterium Anabaena, patS and hetN encode peptide-derived signals with many of the properties of morphogens. These signals regulate the formation of a periodic pattern of heterocysts by lateral inhibition of differentiation. Here we show that intercellular transfer of the patS- and hetN-dependent developmental signals from heterocysts to vegetative cells requires HetC, a predicted ATP-binding cassette transporter (ABC transporter). Relative to the wild type, in a hetC mutant differentiation resulted in a reduced number of heterocysts that were incapable of nitrogen fixation, but deletion of patS or hetN restored heterocyst number and function in a hetC background. These epistasis results suggest that HetC is necessary for conferring self-immunity to the inhibitors on differentiating cells. Nine hours after induction of differentiation, HetC was required for neither induction of transcription of patS nor intercellular transfer of the patS-encoded signal to neighboring cells. Conversely, in strains lacking HetC, the patS- and hetN-encoded signals were not transferred from heterocyst cells to adjacent vegetative cells. The results support a model in which the patS-dependent signal is initially transferred between vegetative cells in a HetC-independent fashion, but some time before morphological differentiation of heterocysts is complete, transfer of both signals transitions to a HetC-dependent process.

IMPORTANCE

How chemical cues that regulate pattern formation in multicellular organisms move from one cell to another is a central question in developmental biology. In this study, we show that an ABC transporter, HetC, is necessary for transport of two developmental signals between different types of cells in a filamentous cyanobacterium. ABC transporters are found in organisms as diverse as bacteria and humans and, as the name implies, are often involved in the transport of molecules across a cellular membrane. The activity of HetC was shown to be required for signaling between heterocysts, which supply fixed nitrogen to the organism, and other cells, as well as for conferring immunity to self-signaling on developing heterocysts.

Spatial fluctuations in expression of the heterocyst differentiation regulatory gene hetR in Anabaena filaments

Under nitrogen deprivation, filaments of the cyanobacterium Anabaena undergo a process of development, resulting in a one-dimensional pattern of nitrogen-fixing heterocysts separated by about ten photosynthetic vegetative cells. Many aspects of gene expression before nitrogen deprivation and during the developmental process remain to be elucidated. Furthermore, the coupling of gene expression fluctuations between cells along a multicellular filament is unknown. We studied the statistics of fluctuations of gene expression of HetR, a transcription factor essential for heterocyst differentiation, both under steady-state growth in nitrogen-rich conditions and at different times following nitrogen deprivation, using a chromosomally-encoded translational hetR-gfp fusion. Statistical analysis of fluorescence at the individual cell level in wild-type and mutant filaments demonstrates that expression fluctuations of hetR in nearby cells are coupled, with a characteristic spatial range of circa two to three cells, setting the scale for cellular interactions along a filament. Correlations between cells predominantly arise from intercellular molecular transfer and less from cell division. Fluctuations after nitrogen step-down can build up on those under nitrogen-replete conditions. We found that under nitrogen-rich conditions, basal, steady-state expression of the HetR inhibitor PatS, cell-cell communication influenced by the septal protein SepJ and positive HetR auto-regulation are essential determinants of fluctuations in hetR expression and its distribution along filaments. A comparison between the expression of hetR-gfp under nitrogen-rich and nitrogen-poor conditions highlights the differences between the two HetR inhibitors PatS and HetN, as well as the differences in specificity between the septal proteins SepJ and FraC/FraD. Activation, inhibition and cell-cell communication lie at the heart of developmental processes. Our results show that proteins involved in these basic ingredients combine together in the presence of inevitable stochasticity in gene expression, to control the coupled fluctuations of gene expression that give rise to a one-dimensional developmental pattern in this organism.

Under prolonged nitrogen deprivation, one-dimensional filaments of the multicellular cyanobacterium Anabaena undergo a process of development, forming a pattern consisting of cells specialized for nitrogen fixation-heterocysts-, separated by a chain of about ten photosynthetic vegetative cells. The developmental program uses activation, inhibition, and transport to create spatial and temporal patterns of gene expression, in the presence of unavoidable stochastic fluctuations in gene expression among cells. Using a chromosomally-encoded fluorescent marker, we followed the expression of the important regulator HetR in individual cells along filaments, both under abundant nitrogen conditions as well as at different times after nitrogen deprivation. The results of our statistical analysis of these fluctuations illuminate the fundamental role that positive feedback, lateral inhibition and cell-cell communication play in the developmental program, not only after exposure to the external cue that triggers differentiation but also under non-inducing conditions. Furthermore our results establish the spatial extent to which gene expression is correlated along filaments.

Regulation of Genes Involved in Heterocyst Differentiation in the Cyanobacterium Anabaena sp. Strain PCC 7120 by a Group 2 Sigma Factor SigC

The filamentous cyanobacterium Anabaena sp. strain PCC 7120 differentiates specialized cells for nitrogen fixation called heterocysts upon limitation of combined nitrogen in the medium. During heterocyst differentiation, expression of approximately 500 genes is upregulated with spatiotemporal regulation. In the present study, we investigated the functions of sigma factors of RNA polymerase in the regulation of heterocyst differentiation. The transcript levels of sigC, sigE, and sigG were increased during heterocyst differentiation, while expression of sigJ was downregulated. We carried out DNA microarray analysis to identify genes regulated by SigC, SigE, and SigG. It was indicated that SigC regulated the expression of genes involved in heterocyst differentiation and functions. Moreover, genes regulated by SigC partially overlapped with those regulated by SigE, and deficiency of SigC was likely to be compensated by SigE.

The trpE gene negatively regulates differentiation of heterocysts at the level of induction in Anabaena sp. strain PCC 7120

Levels of 2-oxoglutarate (2-OG) reflect nitrogen status in many bacteria. In heterocystous cyanobacteria, a spike in the 2-OG level occurs shortly after the removal of combined nitrogen from cultures and is an integral part of the induction of heterocyst differentiation. In this work, deletion of one of the two annotated trpE genes in Anabaena sp. strain PCC 7120 resulted in a spike in the 2-OG level and subsequent differentiation of a wild-type pattern of heterocysts when filaments of the mutant were transferred from growth on ammonia to growth on nitrate. In contrast, 2-OG levels were unaffected in the wild type, which did not differentiate under the same conditions. An inverted-repeat sequence located upstream of trpE bound a central regulator of differentiation, HetR, in vitro and was necessary for HetR-dependent transcription of a reporter fusion and complementation of the mutant phenotype in vivo. Functional complementation of the mutant phenotype with the addition of tryptophan suggested that levels of tryptophan, rather than the demonstrated anthranilate synthase activity of TrpE, mediated the developmental response of the wild type to nitrate. A model is presented for the observed increase in 2-OG in the trpE mutant.

Relationships between the ABC-exporter HetC and peptides that regulate the spatiotemporal pattern of heterocyst distribution in Anabaena

In the model cyanobacterium Anabaena sp. PCC 7120, cells called heterocysts that are specialized in the fixation of atmospheric nitrogen differentiate from vegetative cells of the filament in the absence of combined nitrogen. Heterocysts follow a specific distribution pattern along the filament, and a number of regulators have been identified that influence the heterocyst pattern. PatS and HetN, expressed in the differentiating cells, inhibit the differentiation of neighboring cells. At least PatS appears to be processed and transferred from cell to cell. HetC is similar to ABC exporters and is required for differentiation. We present an epistasis analysis of these regulatory genes and of genes, hetP and asr2819, successively downstream from hetC, and we have studied the localization of HetC and HetP by use of GFP fusions. Inactivation of patS, but not of hetN, allowed differentiation to proceed in a hetC background, whereas inactivation of hetC in patS or patS hetN backgrounds decreased the frequency of contiguous proheterocysts. A HetC-GFP protein is localized to the heterocysts and especially near their cell poles, and a putative HetC peptidase domain was required for heterocyst differentiation but not for HetC-GFP localization. hetP is also required for heterocyst differentiation. A HetP-GFP protein localized mostly near the heterocyst poles. ORF asr2819, which we denote patC, encodes an 84-residue peptide and is induced upon nitrogen step-down. Inactivation of patC led to a late spreading of the heterocyst pattern. Whereas HetC and HetP appear to have linked functions that allow heterocyst differentiation to progress, PatC may have a role in selecting sites of differentiation, suggesting that these closely positioned genes may be functionally related.

Transcriptional dynamics of developmental genes assessed with an FMN-dependent fluorophore in mature heterocysts of Anabaena sp. strain PCC 7120

Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium that differentiates nitrogen-fixing heterocysts when available combined nitrogen is limiting. Growth under diazotrophic conditions results in a mixture of 'new' (recently differentiated) and 'old' (mature) heterocysts. The microoxic environment present in heterocysts makes the interpretation of gene expression using oxygen-dependent fluorophores, including GFP, difficult. The work presented here evaluates the transcriptional dynamics of three developmental genes in mature heterocysts utilizing EcFbFP, a flavin mononucleotide-dependent fluorophore, as the reporter. Expression of both GFP and EcFbFP from the heterologous petE promoter showed that, although GFP and EcFbFP fluoresced in both vegetative cells and new heterocysts, only EcFbFP fluoresced in old heterocysts. A transcriptional fusion of EcFbFP to the late-stage heterocyst-specific nifB promoter displayed continued expression beyond the cessation of GFP fluorescence in heterocysts. Promoter fusions of the master regulator of differentiation, hetR, and its inhibitors, patS and hetN, to GFP and EcFbFP were visualized to determine their role(s) in heterocyst function after morphogenesis. The expression of hetR and hetN was found to persist beyond the completion of development in most heterocysts, whereas patS expression ceased. These data are consistent with a model of heterocyst patterning in which patS is involved in de novo pattern formation, hetN is required for pattern maintenance, and hetR is needed for all stages of development.

Subcellular localization and clues for the function of the HetN factor influencing heterocyst distribution in Anabaena sp. strain PCC 7120

In the filamentous cyanobacterium Anabaena sp. strain PCC 7120, heterocysts are formed in the absence of combined nitrogen, following a specific distribution pattern along the filament. The PatS and HetN factors contribute to the heterocyst pattern by inhibiting the formation of consecutive heterocysts. Thus, inactivation of any of these factors produces the multiple contiguous heterocyst (Mch) phenotype. Upon N stepdown, a HetN protein with its C terminus fused to a superfolder version of green fluorescent protein (sf-GFP) or to GFP-mut2 was observed, localized first throughout the whole area of differentiating cells and later specifically on the peripheries and in the polar regions of mature heterocysts, coinciding with the location of the thylakoids. Polar localization required an N-terminal stretch comprising residues 2 to 27 that may represent an unconventional signal peptide. Anabaena strains expressing a version of HetN lacking this fragment from a mutant gene placed at the native hetN locus exhibited a mild Mch phenotype. In agreement with previous results, deletion of an internal ERGSGR sequence, which is identical to the C-terminal sequence of PatS, also led to the Mch phenotype. The subcellular localization in heterocysts of fluorescence resulting from the fusion of GFP to the C terminus of HetN suggests that a full HetN protein is present in these cells. Furthermore, the full HetN protein is more conserved among cyanobacteria than the internal ERGSGR sequence. These observations suggest that HetN anchored to thylakoid membranes in heterocysts may serve a function besides that of generating a regulatory (ERGSGR) peptide.

Modelling from the experimental developmental biologists viewpoint

In this review we consider Reaction-Diffusion as the archetype of a model in developmental biology. We consider its history in relation to experimental work since it was first proposed in 1952 by Turing and revived in the 1970s by Meinhardt. We then discuss the most recent examples of experiments that address this model, including the challenges that remain in capturing the physico-chemical manifestation of the model mechanism in a real developmental system. Finally we discuss the model's current status and use in the experimental community.