Molecular Diffusion through Cyanobacterial Septal Junctions - PubMed (original) (raw)

Molecular Diffusion through Cyanobacterial Septal Junctions

Mercedes Nieves-Morión et al. mBio. 2017.

Abstract

Heterocyst-forming cyanobacteria grow as filaments in which intercellular molecular exchange takes place. During the differentiation of N2-fixing heterocysts, regulators are transferred between cells. In the diazotrophic filament, vegetative cells that fix CO2 through oxygenic photosynthesis provide the heterocysts with reduced carbon and heterocysts provide the vegetative cells with fixed nitrogen. Intercellular molecular transfer has been traced with fluorescent markers, including calcein, 5-carboxyfluorescein, and the sucrose analogue esculin, which are observed to move down their concentration gradient. In this work, we used fluorescence recovery after photobleaching (FRAP) assays in the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 to measure the temperature dependence of intercellular transfer of fluorescent markers. We find that the transfer rate constants are directly proportional to the absolute temperature. This indicates that the "septal junctions" (formerly known as "microplasmodesmata") linking the cells in the filament allow molecular exchange by simple diffusion, without any activated intermediate state. This constitutes a novel mechanism for molecular transfer across the bacterial cytoplasmic membrane, in addition to previously characterized mechanisms for active transport and facilitated diffusion. Cyanobacterial septal junctions are functionally analogous to the gap junctions of metazoans.

Importance: Although bacteria are frequently considered just as unicellular organisms, there are bacteria that behave as true multicellular organisms. The heterocyst-forming cyanobacteria grow as filaments in which cells communicate. Intercellular molecular exchange is thought to be mediated by septal junctions. Here, we show that intercellular transfer of fluorescent markers in the cyanobacterial filament has the physical properties of simple diffusion. Thus, cyanobacterial septal junctions are functionally analogous to metazoan gap junctions, although their molecular components appear unrelated. Like metazoan gap junctions, the septal junctions of cyanobacteria allow the rapid intercellular exchange of small molecules, without stringent selectivity. Our finding expands the repertoire of mechanisms for molecular transfer across the plasma membrane in prokaryotes.

Copyright © 2017 Nieves-Morión et al.

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Figures

FIG 1

FIG 1

Effect of temperature on the intercellular transfer of calcein, 5-CF, and esculin in Anabaena (Arrhenius plots). BG11-grown filaments were used in FRAP assays as described in “Methods.” The assay temperature was set at 10, 20, 30, or 37°C. Observed, experimental data (mean and standard deviation of 26 to 43 filaments subjected to FRAP analysis for each marker and temperature, except that 43 to 126 filaments were tested at 30°C); Expected, values calculated assuming a _Q_10 of 2, taking as reference the experimental value at 30°C. FRAP data are presented as the recovery rate constant, R (s−1).

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