Outer membrane continuity and septosome formation between vegetative cells in the filaments of Anabaena sp. PCC 7120

Abstract

SummaryAnabaena sp. PCC 7120 is a prototype filamentous nitrogen-fixing cyanobacterium, in which nitrogen fixation and photosynthesis are spatially separated. Recent molecular and cellular studies have established the importance of molecular exchange between cells in the filament, but the routes involved are still under investigation. Two current models propose either a continuous periplasm or direct connections between adjacent cells whose integrity requires the protein SepJ. We used electron tomography to analyze the ultrastructure of the septum between vegetative cells in the Anabaena filament and were able to visualize intercellular connections that we term 'SEPTOSOMES'. We observed that, whereas the existence of the septosome does not depend on the presence of SepJ, the spacing between the two plasma membranes of the septum was significantly decreased in a DsepJ mutant. In addition, we observed that the peptidoglycan layer of each cell enters the septum but the outer membrane does not. Thus, each cell in the filament is individually surrounded by a plasma membrane and a peptidoglycan layer, and physical cell-cell contacts are mediated by the septosome.

Requirement of Fra proteins for communication channels between cells in the filamentous nitrogen-fixing cyanobacteriumAnabaenasp. PCC 7120

“…Using cryo-fixation and staining with osmium tetroxide, which highlights peptidoglycan, and two-axis tomograms, we observed channels with similar length in WT and CSVM34 although there were fewer channels in CSVM34. This difference in observations suggests that different structures connecting cells might be revealed when using cryo-fixation and tomography compared with the structures observed in cells fixed with potassium permanganate (12). However, in agreement with our observations, strain CSVM34 shows a reduced number of septal peptidoglycan nanopores (15% of the wild type) (31).…”

“…Tomograms for the septa between vegetative cells of CSVM34 (ΔfraG), using cells fixed with potassium permanganate, were previously analyzed (12). Structures called "septosomes" were measured to be 18 nm long in CSVM34 compared with 27 nm in WT.…”

“…Two pathways have been proposed for such exchanges (1,(8)(9)(10). One is through the periplasm, suggested by the continuity of the outer membrane surrounding the entire filament (9,11,12). The other proposed means of communication requires structures between adjacent cells in the filament.…”

“…Several structures connecting vegetative cells and heterocysts and vegetative cells with each other have been observed using freeze-fracture, conventional electron microscopy and cryo fixation with electron tomography (13)(14)(15)(16)(17). Different names have been given to these structures: microplasmodesmata, septosomes, septal junctions, or nanopores (12,13,18,19). Using cryopreservation combined with electron tomography, we observed structures we call "channels" traversing the peptidoglycan layer in Anabaena (20).…”

Cluster of Genes That Encode Positive and Negative Elements Influencing Filament Length in a Heterocyst-Forming Cyanobacterium

“…The cyanobacteria are diderm bacteria (10), carrying an outer membrane outside the cytoplasmic membrane and the peptidoglycan layer (11). In filamentous cyanobacteria, whereas the cytoplasmic membrane and peptidoglycan layer surround each cell, the outer membrane is continuous along the filament, defining a continuous periplasm (12)(13)(14)(15).…”