Abstract
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.
Highlights
Heterocyst-forming cyanobacteria form chains of cells whose growth under conditions of deprivation of combined nitrogen depends on the activity of two cell types: vegetative cells that fix CO2 performing oxygenic photosynthesis, and heterocysts devoted to N2 fixation
Heterocysts are formed by differentiation of vegetative cells, which takes place in response to nitrogen deprivation or, in diazotrophic filaments, when the distance between two heterocysts increases as the result of vegetative cell growth
In the mature diazotrophic filament, heterocysts are provided with reduced carbon by the vegetative cells and, in turn, vegetative cells are provided with fixed nitrogen by the heterocysts [8]
Summary
Heterocyst-forming cyanobacteria form chains of cells (filaments or trichomes) whose growth under conditions of deprivation of combined nitrogen depends on the activity of two cell types: vegetative cells that fix CO2 performing oxygenic photosynthesis, and heterocysts devoted to N2 fixation. Heterocysts are formed by differentiation of vegetative cells, which takes place in response to nitrogen deprivation or, in diazotrophic filaments, when the distance between two heterocysts increases as the result of vegetative cell growth. [9,10,11], and and glutamine glutamine and and the the dipeptide dipeptide β-aspartyl β-aspartyl arginine transferred from from heterocysts heterocysts to vegetative cells [10,12]. We review these initial findings and in intercellular molecular transfer [15,16,17]. Developments that that led led to to the the hypothesis hypothesis that that proteinaceous proteinaceous structures structures termed termed “septal mediate molecular exchange in heterocyst-forming cyanobacteria. We furtherWe reevaluate mediate intercellular intercellular molecular exchange in heterocyst-forming cyanobacteria.
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