title: Intercellular diffusion of a fluorescent sucrose analog via the septal junctions in a filamentous cyanobacterium.
abstract: Many filamentous cyanobacteria produce specialized nitrogen-fixing cells called heterocysts, which are located at semiregular intervals along the filament with about 10 to 20 photosynthetic vegetative cells in between. Nitrogen fixation in these complex multicellular bacteria depends on metabolite exchange between the two cell types, with the heterocysts supplying combined-nitrogen compounds but dependent on the vegetative cells for photosynthetically produced carbon compounds. Here, we used a fluorescent tracer to probe intercellular metabolite exchange in the filamentous heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. We show that esculin, a fluorescent sucrose analog, is incorporated by a sucrose import system into the cytoplasm of Anabaena cells. The cytoplasmic esculin is rapidly and reversibly exchanged across vegetative-vegetative and vegetative-heterocyst cell junctions. Our measurements reveal the kinetics of esculin exchange and also show that intercellular metabolic communication is lost in a significant fraction of older heterocysts. SepJ, FraC, and FraD are proteins located at the intercellular septa and are suggested to form structures analogous to gap junctions. We show that a ΔsepJ ΔfraC ΔfraD triple mutant shows an altered septum structure with thinner septa but a denser peptidoglycan layer. Intercellular diffusion of esculin and fluorescein derivatives is impaired in this mutant, which also shows a greatly reduced frequency of nanopores in the intercellular septal cross walls. These findings suggest that FraC, FraD, and SepJ are important for the formation of junctional structures that constitute the major pathway for feeding heterocysts with sucrose.
  
  
  OBJECTIVE
  Anabaena and its relatives are filamentous cyanobacteria that exhibit a sophisticated form of prokaryotic multicellularity, with the formation of differentiated cell types, including normal photosynthetic cells and specialized nitrogen-fixing cells called heterocysts. The question of how heterocysts communicate and exchange metabolites with other cells in the filament is key to understanding this form of bacterial multicellularity. Here we provide the first information on the intercellular exchange of a physiologically important molecule, sucrose. We show that a fluorescent sucrose analog can be imported into the Anabaena cytoplasm by a sucrose import system. Once in the cytoplasm, it is rapidly and reversibly exchanged among all of the cells in the filament by diffusion across the septal junctions. Photosynthetically produced sucrose likely follows the same route from cytoplasm to cytoplasm. We identify some of the septal proteins involved in sucrose exchange, and our results indicate that these proteins form structures functionally analogous to metazoan gap junctions.
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom.
authors.forename: Dennis J
authors.initials: DJ
authors.lastname: Nürnberg
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Consejo Superior de Investigaciones Cientíﬁcas and Universidad de Sevilla, Seville, Spain.
authors.forename: Vicente
authors.initials: V
authors.lastname: Mariscal
authors.affiliation: Department of Microbiology/Organismic Interactions, University of Tübingen, Tübingen, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Bornikoel
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Consejo Superior de Investigaciones Cientíﬁcas and Universidad de Sevilla, Seville, Spain.
authors.forename: Mercedes
authors.initials: M
authors.lastname: Nieves-Morión
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom.
authors.forename: Norbert
authors.initials: N
authors.lastname: Krauß
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Consejo Superior de Investigaciones Cientíﬁcas and Universidad de Sevilla, Seville, Spain.
authors.forename: Antonia
authors.initials: A
authors.lastname: Herrero
authors.affiliation: Department of Microbiology/Organismic Interactions, University of Tübingen, Tübingen, Germany c.mullineaux@qmul.ac.uk eflores@ibvf.csic.es iris.maldener@mikrobio.uni-tuebingen.de.
authors.forename: Iris
authors.initials: I
authors.lastname: Maldener
authors.affiliation: Instituto de Bioquímica Vegetal y Fotosíntesis, Consejo Superior de Investigaciones Cientíﬁcas and Universidad de Sevilla, Seville, Spain c.mullineaux@qmul.ac.uk eflores@ibvf.csic.es iris.maldener@mikrobio.uni-tuebingen.de.
authors.forename: Enrique
authors.initials: E
authors.lastname: Flores
authors.affiliation: School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom c.mullineaux@qmul.ac.uk eflores@ibvf.csic.es iris.maldener@mikrobio.uni-tuebingen.de.
authors.forename: Conrad W
authors.initials: CW
authors.lastname: Mullineaux
mesh_terms: D017033:Anabaena / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D003593:Cytoplasm / Q000737:chemistry; D004058:Diffusion; D004929:Esculin / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D004929:Esculin