Biosynthesis of a sulfated exopolysaccharide, synechan, and bloom formation in the model cyanobacterium Synechocystis sp. strain PCC 6803
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Evaluation Summary:
The authors have elucidated the biochemical and regulatory apparatus for the biosynthesis of sulfated exopolysaccharides, an entire class of molecules not previously studied in cyanobacteria. The work has broad implications for the microbiology and ecology of these organisms and also opens the possibility to use these compounds in biotechnology and modify their structures by combinatorial synthesis.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1, Reviewer #2 and Reviewer #3 agreed to share their names with the authors.)
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- Microbiology and Infectious Disease (eLife)
Abstract
Extracellularpolysaccharides of bacteria contribute to biofilm formation, stress tolerance, and infectivity. Cyanobacteria, the oxygenic photoautotrophic bacteria, uniquely produce sulfated extracellular polysaccharides among bacteria to support phototrophic biofilms. In addition, sulfated polysaccharides of cyanobacteria and other organisms have been focused as beneficial biomaterial. However, very little is known about their biosynthesis machinery and function in cyanobacteria. Here, we found that the model cyanobacterium, Synechocystis sp. strain PCC 6803, formed bloom-like cell aggregates embedded in sulfated extracellular polysaccharides (designated as synechan) and identified whole set of genes responsible for synechan biosynthesis and its transcriptional regulation, thereby suggesting a model for the synechan biosynthesis apparatus. Because similar genes are found in many cyanobacterial genomes with wide variation, our findings may lead elucidation of various sulfated polysaccharides, their functions, and their potential application in biotechnology.
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Reviewer #3 (Public Review):
The manuscript entitled "Biosynthesis system of Synechan, a sulfated exopolysaccharide, in the model cyanobacterium Synechocystis sp. PCC 6803" is a scientifically sound manuscript and is of interest for a broad scientific audience. It provides interesting and valuable new insights and many experiments were performed. However, there are some points which must be addressed to make the manuscript more consistent and easier to grasp.
- Title: I would suggest to change the title, since Biosynthesis system is not a common term.
- Abstract: Cyanbobacteria are not unique in having sulphated polysaccharides. What is about Carrageenan's and also exopolysaccharides from Porphyridium strains (see current publications on that). If it means that amongst bacteria the cyanobacteria are the only ones, this should be clearly …
Reviewer #3 (Public Review):
The manuscript entitled "Biosynthesis system of Synechan, a sulfated exopolysaccharide, in the model cyanobacterium Synechocystis sp. PCC 6803" is a scientifically sound manuscript and is of interest for a broad scientific audience. It provides interesting and valuable new insights and many experiments were performed. However, there are some points which must be addressed to make the manuscript more consistent and easier to grasp.
- Title: I would suggest to change the title, since Biosynthesis system is not a common term.
- Abstract: Cyanbobacteria are not unique in having sulphated polysaccharides. What is about Carrageenan's and also exopolysaccharides from Porphyridium strains (see current publications on that). If it means that amongst bacteria the cyanobacteria are the only ones, this should be clearly stated.
- Would avoid to use may utilize the polysaccharides... Please be more specific or delete this.
- Lane 32: Can really every bacterium produce several EPS? This should be carefully evaluated.
- Lane 34: The applications named are very broad and not specific, what are the real applications there?
- Lane 49: again uniquely?
- Lane 56: the sulphated polysaccharides are used for colony and biofilm... This sentence must be rephrased and corrected.
- Lane 84: bubbling culture etc. I can´t find any detailed explanation on the cultivation systems, what is essential for the methods part. Please add volume, light source and principle of illumination (inside outside etc.). Please rephrase the sentence that the light was generated by fluorescent lamps.... They were used for illumination.
- Lane 97: GTs can not be screened by disruption, it is their function what is screened.
- Figure: would suggest to use A) instead of A,
- Table S1: What does Importance mean in the table, would suggest to change that towards a more specific value/information
- Lane 232: to see the transcriptome... this should be rephrased
- The description of the different EPS is a bit confusing, since it is only described that the WT contains several sugars, which are then given in table 1. The deletion strain shows a different composition. This should be explained a bit straighter. Why is ribose given in table 1, if there is no ribose observed? In general, the whole manuscript needs correction of the English language to make it clearer in some aspects. Also, the structure of the manuscript might be reworked a bit, since it is a bit confusing in some parts. Especially the effect of the different deletions should be given clearly and straight. Also, the complexity of the manuscript will be easier to grasp by some rearrangements of the results. The current complexity might come from having all supplement figures already in the manuscript, but it also comes from sometimes complex sentences, as well as jumping a bit in between the topics. But finally, this is a really valuable and interesting study.
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Reviewer #2 (Public Review):
The paper does a very thorough job of identifying genes important for the production and export of a sulfated exopolysaccharide in Synechocystis, leading to a clear and well-justified model for EPS production and its regulation. The authors also make a convincing case for the importance of EPS production for the formation of floating multicellular aggregrates or "blooms". However, the relationship between EPS production and bloom formation is not quantitative (some mutants show markedly reduced EPS production without any discernible effect on bloom formation) which indicates that bloom formation must involve additional factors which are not currently discussed.
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Reviewer #1 (Public Review):
The authors have identified an entire set of genes for the synthesis of sulfated exopolysaccharides (EPS) in the cyanobacterial model Synechocystis 6803. They show convincingly that the respective gene products are involved in the production of these compounds and they have extensively characterized the regulation of these genes. Among the regulators they found a STAND protein. STAND proteins include animal and plant regulators of programmed cell death but were rarely characterized in bacteria. Last but not least they come up with an entirely new model for the buoyancy regulation of cyanobacteria (as light-dependent aquatic organisms it is important for cyanobacteria where they are in the water column). The authors suggest a mechanism in which EPS-entrapped cells together with extracellular gas bubbles …
Reviewer #1 (Public Review):
The authors have identified an entire set of genes for the synthesis of sulfated exopolysaccharides (EPS) in the cyanobacterial model Synechocystis 6803. They show convincingly that the respective gene products are involved in the production of these compounds and they have extensively characterized the regulation of these genes. Among the regulators they found a STAND protein. STAND proteins include animal and plant regulators of programmed cell death but were rarely characterized in bacteria. Last but not least they come up with an entirely new model for the buoyancy regulation of cyanobacteria (as light-dependent aquatic organisms it is important for cyanobacteria where they are in the water column). The authors suggest a mechanism in which EPS-entrapped cells together with extracellular gas bubbles derived from photosynthesis form multicellular complexes that float at certain depths. This would be a very important function and explain the extensive regulatory and signaling apparatus in controlling the synthesis of these sulfated EPS.
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Evaluation Summary:
The authors have elucidated the biochemical and regulatory apparatus for the biosynthesis of sulfated exopolysaccharides, an entire class of molecules not previously studied in cyanobacteria. The work has broad implications for the microbiology and ecology of these organisms and also opens the possibility to use these compounds in biotechnology and modify their structures by combinatorial synthesis.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1, Reviewer #2 and Reviewer #3 agreed to share their names with the authors.)
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