Molecular Diffusion through Cyanobacterial Septal Junctions

Mercedes Nieves-Morión,Enrique Flores,Conrad W Mullineaux,Stephen J Giovannoni

doi:10.1128/mbio.01756-16

Abstract

ABSTRACTHeterocyst-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.

Highlights

Heterocyst-forming cyanobacteria grow as filaments in which intercellular molecular exchange takes place
We show that intercellular transfer of fluorescent markers in the cyanobacterial filament has the physical properties of simple diffusion
As evidenced by the use of fluorescent markers, including calcein [8], 5-carboxyfluorescein (5-CF) [9], and the sucrose analogue esculin [6], intercellular molecular exchange takes place between the vegetative cells of diazotrophic filaments and of filaments grown in the presence of combined nitrogen

Summary

Introduction

Heterocyst-forming cyanobacteria grow as filaments in which intercellular molecular exchange takes place. 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. As evidenced by the use of fluorescent markers, including calcein [8], 5-carboxyfluorescein (5-CF) [9], and the sucrose analogue esculin [6], intercellular molecular exchange takes place between the vegetative cells of diazotrophic filaments and of filaments grown in the presence of combined nitrogen. SepJ and FraD have long extramembrane domains that, as discussed previously [12], may reside in the periplasmic area of the intercellular septa These proteins are necessary for the formation of septal peptidoglycan nanopores [6], through which septal junctions appear to traverse the septal peptidoglycan [17]. Structures observed by electron tomography of Anabaena that have been termed “channels” [18, 19] likely correspond to the nanopores

Methods

Conclusion