How multicellular cyanobacteria transport molecules

July 12, 2019

Also known as blue-green algae, cyanobacteria are a special class of bacteria that are able to perform photosynthesis. In evolutionary terms, they are ancient. Their predecessors - which first emerged on earth some 2.5 billion years ago - paved the way for higher forms of life thanks to their ability to produce oxygen by photosynthesis.

Some cyanobacterial species are filamentous, multicellular organisms that have developed differentiated cell functions. Some cells perform photosynthesis, while others absorb atmospheric nitrogen. The cyanobacteria obtain energy in the form of glucose through photosynthesis; they use the nitrogen to produce amino acids, the building blocks of proteins.

For the cyanobacteria, this poses the problem of how the individual cells can communicate and exchange substances. The photosynthetic cells have to keep their nitrogen-fixing sister cells supplied with glucose; by the same token, amino acids need to be transported in the opposite direction. To this end, cyanobacteria have developed special cell junctions that permit the exchange of nutrients and messengers across cell boundaries, without the cells being fused together.

Elucidating the structure in cellular context

To date, very little was known about the detailed structure and precise functioning of the cell junctions in multicellular, filamentous cyanobacteria. In the latest issue of the scientific journal Cell, a group of researchers from ETH Zurich and the University of Tübingen presents an unprecedented degree of detail on the structure and function of the cell-to-cell connections, referred to as septal junctions, in the Anabaena genus.

The researchers reveal that the connecting channels are composed of a protein tube that is sealed with a plug at both ends. Moreover, this tube is covered with five-armed protein elements, which are arranged much like a camera aperture.

The channels connect the cytoplasms of two neighbouring cells by passing through the different membranes and cell walls. The cells are separated by an ultra-thin gap, just a few nanometres wide.

"Researchers have so far failed to clarify these details with conventional electron microscopy. By extending cryo-electron microscopy, we were able to gain a degree of precision never before achieved," says Professor Martin Pilhofer from the Institute of Molecular Biology and Biophysics at ETH Zurich.

Gregor Weiss, Pilhofer's doctoral student, developed a process of preparing the cyanobacteria in such a way that the channels could be visualised via cryo-electron microscopy. Using frozen cyanobacteria, Weiss "milled" the junction between two cells, layer by layer, until his sample was thin enough. Without this pre-processing, the spherical cells would have been too thick for cryo-electron microscopy.

Mechanism to prevent leaking

"Due to the complex structure of the connecting channels, we suspected there was a mechanism to open and close them," said Karl Forchhammer, Professor for Microbiology at the University of Tübingen. He and his team were in fact able to show how the cells of the complex communicate with each other under different stress conditions. They stained cyanobacteria chains with a fluorescent dye and then bleached individual cells with a laser. The researchers then measured the influx of the dye from neighbouring cells.

Using this method, they were able to show that the channels actually close when treated with chemicals or in the dark. The filigree cap structure of a channel closes like an iris and interrupts the exchange of substances between the cells; the researchers recognised this phenomenon through the varying degree of fluorescence they observed.

"This closing mechanism protects the entire multicellular organism," Forchhammer says. For example, it can prevent a cell from passing on harmful substances to its neighbouring cells, which could destroy the whole organism. The cyanobacteria can also use the channels to prevent the cell contents of the entire network from leaking out if individual cells are mechanically damaged.

Conserved structures

With their study, the researchers are able to show that in the course of evolution, multicellular organisms of different lineages repeatedly and independently "invented" cell junctions. "It emphasises just how important it is for a multicellular organism to be able to monitor the transport of substances between its individual cells," Pilhofer says. By elucidating the channel structure and function in cyanobacteria, the ETH researchers are adding another piece to the puzzle. "As far as we are concerned, this is fundamental biological research, without focusing on any potential application. The new data rather gives us a greater understanding of the evolution of complex life forms," the ETH professor explains.

Weiss GL, Kieninger A-K, Maldener I, Forchhammer K, Pilhofer M. Structure and function of a bacterial gap junction analog. Cell, 2019, July 11th. DOI 10.1016/j.cell.2019.05.055

ETH Zurich

Related Photosynthesis Articles from Brightsurf:

During COVID, scientists turn to computers to understand C4 photosynthesis
When COVID closed down their lab, a team from the University of Essex turned to computational approaches to understand what makes some plants better adapted to transform light and carbon dioxide into yield through photosynthesis.

E. coli bacteria offer path to improving photosynthesis
Cornell University scientists have engineered a key plant enzyme and introduced it in Escherichia coli bacteria in order to create an optimal experimental environment for studying how to speed up photosynthesis, a holy grail for improving crop yields.

Showtime for photosynthesis
Using a unique combination of nanoscale imaging and chemical analysis, an international team of researchers has revealed a key step in the molecular mechanism behind the water splitting reaction of photosynthesis, a finding that could help inform the design of renewable energy technology.

Photosynthesis in a droplet
Researchers develop an artificial chloroplast.

Even bacteria need their space: Squished cells may shut down photosynthesis
Introverts take heart: When cells, like some people, get too squished, they can go into defense mode, even shutting down photosynthesis.

Marine cyanobacteria do not survive solely on photosynthesis
The University of Cordoba published a study in a journal from the Nature group that supports the idea that marine cyanobacteria also incorporate organic compounds from the environment.

Photosynthesis -- living laboratories
Ludwig-Maximilians-Universitaet (LMU) in Munich biologists Marcel Dann and Dario Leister have demonstrated for the first time that cyanobacteria and plants employ similar mechanisms and key proteins to regulate cyclic electron flow during photosynthesis.

Photosynthesis seen in a new light by rapid X-ray pulses
In a new study, led by Petra Fromme and Nadia Zatsepin at the Biodesign Center for Applied Structural Discovery, the School of Molecular Sciences and the Department of Physics at ASU, researchers investigated the structure of Photosystem I (PSI) with ultrashort X-ray pulses at the European X-ray Free Electron Laser (EuXFEL), located in Hamburg, Germany.

Photosynthesis olympics: can the best wheat varieties be even better?
Scientists have put elite wheat varieties through a sort of 'Photosynthesis Olympics' to find which varieties have the best performing photosynthesis.

Strange bacteria hint at ancient origin of photosynthesis
Structures inside rare bacteria are similar to those that power photosynthesis in plants today, suggesting the process is older than assumed.

Read More: Photosynthesis News and Photosynthesis Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to