Cell envelope components influencing filament length in the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120

Cell envelope diversity and evolution across the bacterial tree of life

The bacterial cell envelope is a complex multilayered structure conserved across all bacterial phyla. It is categorized into two main types based on the number of membranes surrounding the cell. Monoderm bacteria are enclosed by a single membrane, whereas diderm cells are distinguished by the presence of a second, outer membrane (OM). An ancient divide in the bacterial domain has resulted in two major clades: the Gracilicutes, consisting strictly of diderm phyla; and the Terrabacteria, encompassing monoderm and diderm species with diverse cell envelope architectures. Recent structural and phylogenetic advancements have improved our understanding of the diversity and evolution of the OM across the bacterial tree of life. Here we discuss cell envelope variability within major bacterial phyla and focus on conserved features found in diderm lineages. Characterizing the mechanisms of OM biogenesis and the evolutionary gains and losses of the OM provides insights into the primordial cell and the last universal common ancestor from which all living organisms subsequently evolved.

Two TonB-dependent outer membrane transporters involved in heme uptake in Anabaena sp. PCC 7120

Low availability of micronutrients such as iron has enforced the evolution of uptake systems in all kingdoms of life. In Gram-negative bacteria, outer membrane, periplasmatic and plasma membrane localized proteins facilitate the uptake of iron-loaded chelators, which are energized by TonB proteins. The specificity of different uptake systems likely depends either on the endogenously produced siderophore or on the bioavailability of iron-chelator complexes in the environment. Hence, an uptake system for schizokinen produced by the model cyanobacterium Anabaena sp. PCC 7120 exists, while bioinformatics analysis suggests the existence of additional systems, likely for uptake of xenosiderophores. Consistently, proteins encoded by alr2153 (hutA1) and alr3242 (hutA2) are assigned as outer membrane heme transporters. Indeed, Anabaena sp. PCC 7120 can utilize external heme as an iron source. The addition of heme resulted in an induction of genes involved in heme degradation and chlorophyll a synthesis and in an increase of the chlorophyll a content. Moreover, iron starvation induced the expression of hutA1, while the addition of heme led to its repression. Remarkably, the addition of a high concentration of heme but not iron starvation resulted in hutA2 induction. Plasmid insertion mutants of both genes exhibited a reduced capacity to recover from iron starvation by heme addition, which indicates a dependence of heme uptake on functional HutA1 and HutA2 proteins. The structural model generated by bioinformatics methods is further in agreement with a role in heme uptake. Thus, we provide evidence that Anabaena sp. PCC 7120 uses a heme uptake system in parallel to other iron acquisition systems.

SepT, a novel protein specific to multicellular cyanobacteria, influences peptidoglycan growth and septal nanopore formation in Anabaena sp. PCC 7120

IMPORTANCE

Multicellular organization is a requirement for the development of complex organisms, and filamentous cyanobacteria such as Anabaena represent a paradigmatic case of bacterial multicellularity. The Anabaena filament can include hundreds of communicated cells that exchange nutrients and regulators and, depending on environmental conditions, can include different cell types specialized in distinct biological functions. Hence, the specific features of the Anabaena filament and how they are propagated during cell division represent outstanding biological issues. Here, we studied SepT, a novel coiled-coil-rich protein of Anabaena that is located in the intercellular septa and influences the formation of the septal specialized structures that allow communication between neighboring cells along the filament, a fundamental trait for the performance of Anabaena as a multicellular organism.

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 Role of Mre Factors and Cell Division in Peptidoglycan Growth in the Multicellular Cyanobacterium Anabaena

Bacteria in general serve two main tasks: cell growth and division. Both processes include peptidoglycan extension to allow cell expansion and to form the poles of the daughter cells, respectively. The cyanobacterium Anabaena forms filaments of communicated cells in which the outer membrane and the peptidoglycan sacculus, which is engrossed in the intercellular regions between contiguous cells, are continuous along the filament. During the growth of Anabaena, peptidoglycan incorporation was weak at the cell periphery. During cell division, midcell peptidoglycan incorporation matched the localization of the divisome, and incorporation persisted in the intercellular septa, even after the division was completed. MreB, MreC, and MreD were located throughout the cell periphery and, in contrast to other bacteria, also to the divisome all along midcell peptidoglycan growth. In Anabaena mutants bearing inactivated mreB, mreC, or mreD genes, which showed conspicuous alterations in the filament morphology, consecutive septal bands of peptidoglycan growth were frequently not parallel to each other and were irregularly spaced along the filament, reproducing the disposition of the Z-ring. Both lateral and septal growth was impaired in strains down-expressing Z-ring components, and MreB and MreD appeared to directly interact with some divisome components. We propose that, in Anabaena, association with the divisome is a way for localization of MreB, MreC, and MreD at the cell poles, where they regulate lateral, midcell, and septal peptidoglycan growth with the latter being involved in localization and maintenance of the intercellular septal-junction protein structures that mediate cell-cell communication along the filament.

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.

Functions of the Essential Gene mraY in Cellular Morphogenesis and Development of the Filamentous Cyanobacterium Anabaena PCC 7120

Bacterial cell shape is determined by the peptidoglycan (PG) layer. The cyanobacterium Anabaena sp. PCC 7120 (Anabaena) is a filamentous strain with ovoid-shaped cells connected together with incomplete cell constriction. When deprived of combined nitrogen in the growth medium, about 5–10% of the cells differentiate into heterocysts, cells devoted to nitrogen fixation. It has been shown that PG synthesis is modulated during heterocyst development and some penicillin-binding proteins (PBPs) participating in PG synthesis are required for heterocyst morphogenesis or functioning. Anabaena has multiple PBPs with functional redundancy. In this study, in order to examine the function of PG synthesis and its relationship with heterocyst development, we created a conditional mutant of mraY, a gene necessary for the synthesis of the PG precursor, lipid I. We show that mraY is required for cell and filament integrity. Furthermore, when mraY expression was being limited, persistent septal PG synthetic activity was observed, resulting in increase in cell width. Under non-permissive conditions, filaments and cells were rapidly lysed, and no sign of heterocyst development within the time window allowed was detected after nitrogen starvation. When mraY expression was being limited, a high percentage of heterocyst doublets were found. These doublets are formed likely as a consequence of delayed cell division and persistent septal PG synthesis. MraY interacts with components of both the elongasome and the divisome, in particular those directly involved in PG synthesis, including HetF, which is required for both cell division and heterocyst formation.

Final Destination? Pinpointing Hyella disjuncta sp. nov. PCC 6712 (Cyanobacteria) Based on Taxonomic Aspects, Multicellularity, Nitrogen Fixation and Biosynthetic Gene Clusters

Unicellular cyanobacteria inhabit a wide range of ecosytems and can be found throughout the phylum offering space for taxonomic confusion. One example is strain PCC 6712 that was described as Chlorogloea sp. (Nostocales) and later assigned to the genus Chroococcidiopsis (Chroococcidiopsidales). We now show that this strain belongs to the order Pleurocapsales and term it Hyella disjuncta based on morphology, genome analyses and 16S-23S ITS rRNA phylogeny. Genomic analysis indicated that H. disjuncta PCC 6712 shared about 44.7% orthologue genes with its closest relative H. patelloides. Furthermore, 12 cryptic biosynthetic gene clusters (BGCs) with potential bioactivity, such as a mycosporine-like amino acid BGC, were detected. Interestingly, the full set of nitrogen fixation genes was found in H. disjuncta PCC 6712 despite its inability to grow on nitrogen-free medium. A comparison of genes responsible for multicellularity was performed, indicating that most of these genes were present and related to those found in other cyanobacterial orders. This is in contrast to the formation of pseudofilaments-a main feature of the genus Hyella-which is weakly expressed in H. disjuncta PCC 6712 but prominent in Hyella patelloides LEGE 07179. Thus, our study pinpoints crucial but hidden aspects of polyphasic cyanobacterial taxonomy.

Sheathless Separation of Cyanobacterial Anabaena by Shape Using Viscoelastic Microfluidics

Cyanobacteria have a wide range of impact on natural ecosystems, and have been recognized as potentially rich sources of pharmacological and structurally interesting secondary metabolites. To better understand the basic molecular processes and mechanisms that influence and regulate the growth (like length) of cyanobacteria, or connections between environment, genotype, and phenotype, it would be essential to separate shape-synchronized cyanobacterial cell populations with relatively uniform length and size. This work proposes a novel and efficient method to separate cyanobacterial Anabaena by shape (rod aspect ratio) using viscoelastic microfluidics in a straight channel with expansion-contraction cavity arrays (ECCA channel). The biocompatible viscoelastic solutions with dissolved polymer would induce a combined effect of inertial lift force, elastic force, and secondary drag force for Anabaena flowing in it. Therefore, Anabaena with different lengths reach different lateral equilibrium positions and flow out from different outlets. Factors including flow rate, fluid viscoelasticity, channel structure, and length on the shape-based cell separation were studied systematically. This work, for the first time, demonstrates continuous and sheathless shape-based separation of cyanobacteria using viscoelastic microfluidics. Moreover, its ability to manipulate objects with different morphologies and with a size of >100 μm will extend the capability of microfluidics to a completely new field that has never been reached and would be attractive across a range of new applications.

Impaired cell-cell communication in the multicellular cyanobacterium Anabaena affects carbon uptake, photosynthesis, and the cell wall

Cell-cell communication is an essential attribute of multicellular organisms. The effects of perturbed communication were studied in septal protein mutants of the heterocyst-forming filamentous cyanobacterium Anabaena sp. PCC 7120 model organism. Strains bearing sepJ and sepJ/fraC/fraD deletions showed differences in growth, pigment absorption spectra, and spatial patterns of expression of the hetR gene encoding a heterocyst differentiation master regulator. Global changes in gene expression resulting from deletion of those genes were mapped by RNA sequencing analysis of wild-type and mutant strains, both under nitrogen-replete and nitrogen-poor conditions. The effects of sepJ and fraC/fraD deletions were non-additive, and perturbed cell-cell communication led to significant changes in global gene expression. Most significant effects, related to carbon metabolism, included increased expression of genes encoding carbon uptake systems and components of the photosynthetic apparatus, as well as decreased expression of genes encoding cell wall components related to heterocyst differentiation and to polysaccharide export.

Structural Determinants and Their Role in Cyanobacterial Morphogenesis

Cells have to erect and sustain an organized and dynamically adaptable structure for an efficient mode of operation that allows drastic morphological changes during cell growth and cell division. These manifold tasks are complied by the so-called cytoskeleton and its associated proteins. In bacteria, FtsZ and MreB, the bacterial homologs to tubulin and actin, respectively, as well as coiled-coil-rich proteins of intermediate filament (IF)-like function to fulfil these tasks. Despite generally being characterized as Gram-negative, cyanobacteria have a remarkably thick peptidoglycan layer and possess Gram-positive-specific cell division proteins such as SepF and DivIVA-like proteins, besides Gram-negative and cyanobacterial-specific cell division proteins like MinE, SepI, ZipN (Ftn2) and ZipS (Ftn6). The diversity of cellular morphologies and cell growth strategies in cyanobacteria could therefore be the result of additional unidentified structural determinants such as cytoskeletal proteins. In this article, we review the current advances in the understanding of the cyanobacterial cell shape, cell division and cell growth.

Two novel heteropolymer-forming proteins maintain the multicellular shape of the cyanobacterium Anabaena sp. PCC 7120

Polymerizing and filament-forming proteins are instrumental for numerous cellular processes such as cell division and growth. Their function in stabilization and localization of protein complexes and replicons is achieved by a filamentous structure. Known filamentous proteins assemble into homopolymers consisting of single subunits - for example, MreB and FtsZ in bacteria - or heteropolymers that are composed of two subunits, for example, keratin and α/β tubulin in eukaryotes. Here, we describe two novel coiled-coil-rich proteins (CCRPs) in the filament-forming cyanobacterium Anabaena sp. PCC 7120 (hereafter Anabaena) that assemble into a heteropolymer and function in the maintenance of the Anabaena multicellular shape (termed trichome). The two CCRPs - Alr4504 and Alr4505 (named ZicK and ZacK) - are strictly interdependent for the assembly of protein filaments in vivo and polymerize nucleotide independently in vitro, similar to known intermediate filament (IF) proteins. A ΔzicKΔzacK double mutant is characterized by a zigzagged cell arrangement and hence a loss of the typical linear Anabaena trichome shape. ZicK and ZacK interact with themselves, with each other, with the elongasome protein MreB, the septal junction protein SepJ and the divisome associate septal protein SepI. Our results suggest that ZicK and ZacK function in cooperation with SepJ and MreB to stabilize the Anabaena trichome and are likely essential for the manifestation of the multicellular shape in Anabaena. Our study reveals the presence of filament-forming IF-like proteins whose function is achieved through the formation of heteropolymers in cyanobacteria.

The role of the cytoskeletal proteins MreB and FtsZ in multicellular cyanobacteria

Multiseriate and true‐branching cyanobacteria are at the peak of prokaryotic morphological complexity. However, little is known about the mechanisms governing multiplanar cell division and morphogenesis. Here, we study the function of the prokaryotic cytoskeletal proteins, MreB and FtsZ in Fischerella muscicola PCC 7414 and Chlorogloeopsis fritschii PCC 6912. Vancomycin and HADA labeling revealed a mixed apical, septal, and lateral trichome growth mode in F. muscicola, whereas C. fritschii exhibits septal growth. In all morphotypes from both species, MreB forms either linear filaments or filamentous strings and can interact with FtsZ. Furthermore, multiplanar cell division in F. muscicola likely depends on FtsZ dosage. Our results lay the groundwork for future studies on cytoskeletal proteins in morphologically complex cyanobacteria.

The Inorganic Nutrient Regime and the mre Genes Regulate Cell and Filament Size and Morphology in the Phototrophic Multicellular Bacterium Anabaena

Most studies on the determination of bacterial cell morphology have been conducted in heterotrophic organisms. Here, we present a study of how the availability of inorganic nitrogen and carbon sources influence cell size and morphology in the context of a phototrophic metabolism, as found in the multicellular cyanobacterium Anabaena. In Anabaena, the expression of the MreB, MreC, and MreD proteins, which influence cell size and length, are regulated by NtcA, a transcription factor that globally coordinates cellular responses to the C-to-N balance of the cells. Moreover, MreB, MreC, and MreD also influence septal peptidoglycan construction, thus affecting filament length and, possibly, intercellular molecular exchange that is required for diazotrophic growth. Thus, here we identified new roles for Mre proteins in relation to the phototrophic and multicellular character of a cyanobacterium, Anabaena.

The model cyanobacterium Anabaena sp. PCC 7120 exhibits a phototrophic metabolism relying on oxygenic photosynthesis and a complex morphology. The organismic unit is a filament of communicated cells that may include cells specialized in different nutritional tasks, thus representing a paradigm of multicellular bacteria. In Anabaena, the inorganic carbon and nitrogen regime influenced not only growth, but also cell size, cell shape, and filament length, which also varied through the growth cycle. When using combined nitrogen, especially with abundant carbon, cells enlarged and elongated during active growth. When fixing N₂, which imposed lower growth rates, shorter and smaller cells were maintained. In Anabaena, gene homologs to mreB, mreC, and mreD form an operon that was expressed at higher levels during the phase of fastest growth. In an ntcA mutant, mre transcript levels were higher than in the wild type and, consistently, cells were longer. Negative regulation by NtcA can explain that Anabaena cells were longer in the presence of combined nitrogen than in diazotrophic cultures, in which the levels of NtcA are higher. mreB, mreC, and mreD mutants could grow with combined nitrogen, but only the latter mutant could grow diazotrophically. Cells were always larger and shorter than wild-type cells, and their orientation in the filament was inverted. Consistent with increased peptidoglycan width and incorporation in the intercellular septa, filaments were longer in the mutants, suggesting a role for MreB, MreC, and MreD in the construction of septal peptidoglycan that could affect intercellular communication required for diazotrophic growth.

IMPORTANCE Most studies on the determination of bacterial cell morphology have been conducted in heterotrophic organisms. Here, we present a study of how the availability of inorganic nitrogen and carbon sources influence cell size and morphology in the context of a phototrophic metabolism, as found in the multicellular cyanobacterium Anabaena. In Anabaena, the expression of the MreB, MreC, and MreD proteins, which influence cell size and length, are regulated by NtcA, a transcription factor that globally coordinates cellular responses to the C-to-N balance of the cells. Moreover, MreB, MreC, and MreD also influence septal peptidoglycan construction, thus affecting filament length and, possibly, intercellular molecular exchange that is required for diazotrophic growth. Thus, here we identified new roles for Mre proteins in relation to the phototrophic and multicellular character of a cyanobacterium, Anabaena.

The small Ca2+-binding protein CSE links Ca2+ signalling with nitrogen metabolism and filament integrity in Anabaena sp. PCC 7120

Background

Filamentous cyanobacteria represent model organisms for investigating multicellularity. For many species, nitrogen-fixing heterocysts are formed from photosynthetic vegetative cells under nitrogen limitation. Intracellular Ca²⁺ has been implicated in the highly regulated process of heterocyst differentiation but its role remains unclear. Ca²⁺ is known to operate more broadly in metabolic signalling in cyanobacteria, although the signalling mechanisms are virtually unknown. A Ca²⁺-binding protein called the Ca²⁺ Sensor EF-hand (CSE) is found almost exclusively in filamentous cyanobacteria. Expression of asr1131 encoding the CSE protein in Anabaena sp. PCC 7120 was strongly induced by low CO₂ conditions, and rapidly downregulated during nitrogen step-down. A previous study suggests a role for CSE and Ca²⁺ in regulation of photosynthetic activity in response to changes in carbon and nitrogen availability.

Results

In the current study, a mutant Anabaena sp. PCC 7120 strain lacking asr1131 (Δcse) was highly prone to filament fragmentation, leading to a striking phenotype of very short filaments and poor growth under nitrogen-depleted conditions. Transcriptomics analysis under nitrogen-replete conditions revealed that genes involved in heterocyst differentiation and function were downregulated in Δcse, while heterocyst inhibitors were upregulated, compared to the wild-type.

Conclusions

These results indicate that CSE is required for filament integrity and for proper differentiation and function of heterocysts upon changes in the cellular carbon/nitrogen balance. A role for CSE in transmitting Ca²⁺ signals during the first response to changes in metabolic homeostasis is discussed.

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.

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.

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.

The Peptidoglycan-Binding Protein SjcF1 Influences Septal Junction Function and Channel Formation in the Filamentous Cyanobacterium Anabaena

Filamentous, heterocyst-forming cyanobacteria exchange nutrients and regulators between cells for diazotrophic growth. Two alternative modes of exchange have been discussed involving transport either through the periplasm or through septal junctions linking adjacent cells. Septal junctions and channels in the septal peptidoglycan are likely filled with septal junction complexes. While possible proteinaceous factors involved in septal junction formation, SepJ (FraG), FraC, and FraD, have been identified, little is known about peptidoglycan channel formation and septal junction complex anchoring to the peptidoglycan. We describe a factor, SjcF1, involved in regulation of septal junction channel formation in the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. SjcF1 interacts with the peptidoglycan layer through two peptidoglycan-binding domains and is localized throughout the cell periphery but at higher levels in the intercellular septa. A strain with an insertion in sjcF1 was not affected in peptidoglycan synthesis but showed an altered morphology of the septal peptidoglycan channels, which were significantly wider in the mutant than in the wild type. The mutant was impaired in intercellular exchange of a fluorescent probe to a similar extent as a sepJ deletion mutant. SjcF1 additionally bears an SH3 domain for protein-protein interactions. SH3 binding domains were identified in SepJ and FraC, and evidence for interaction of SjcF1 with both SepJ and FraC was obtained. SjcF1 represents a novel protein involved in structuring the peptidoglycan layer, which links peptidoglycan channel formation to septal junction complex function in multicellular cyanobacteria. Nonetheless, based on its subcellular distribution, this might not be the only function of SjcF1.

Cell-cell communication is central not only for eukaryotic but also for multicellular prokaryotic systems. Principles of intercellular communication are well established for eukaryotes, but the mechanisms and components involved in bacteria are just emerging. Filamentous heterocyst-forming cyanobacteria behave as multicellular organisms and represent an excellent model to study prokaryotic cell-cell communication. A path for intercellular metabolite exchange appears to involve transfer through molecular structures termed septal junctions. They are reminiscent of metazoan gap junctions that directly link adjacent cells. In cyanobacteria, such structures need to traverse the peptidoglycan layers in the intercellular septa of the filament. Here we describe a factor involved in the formation of channels across the septal peptidoglycan layers, thus contributing to the multicellular behavior of these organisms.