Presence of toxin-antitoxin systems in picocyanobacteria and their ecological implications


Picocyanobacteria (mainly Synechococcus and Prochlorococcus) contribute significantly to ocean’s primary production. Toxin-Antitoxin (TA) systems present in bacteria and archaea are known to regulate cell growth in response to environmental stresses. However, little is known about the presence of TA systems in picocyanobacteria. This study investigated complete genomes of Synechococcus and Prochlorococcus to understand the prevalence of TA systems in picocyanobacteria. Using the TAfinder software, Type II TA systems were predicted in 27 of 33 (81%) Synechococcus strains, but none of 38 Prochlorococcus strains contain TA genes. Synechococcus strains with larger genomes tend to contain more putative type II TA systems. The number of TA pairs varies from 0 to 42 in Synechococcus strains isolated from various environments. A linear correlation between the genome size and the number of putative TA systems in both coastal and freshwater Synechococcus was established. In general, open ocean Synechococcus contain no or few TA systems, while coastal and freshwater Synechococcus contain more TA systems. The type II TA systems inhibit microbial translation via ribonucleases and allow cells to enter the “dormant” stage in adverse environments. Inheritance of TA genes in freshwater and coastal Synechococcus could confer a recoverable persister mechanism important to survive in variable environments.

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Fig. 1: Occurrence frequency of putative TA systems in 33 strains of Synechococcus isolated from various aquatic environments.
Fig. 2: Relationship between genome size and the number of putative TA pairs in Synechococcus.
Fig. 3: Conserved domain regions of putative toxins and antitoxins.


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We acknowledge the grant support of U.S. National Science Foundation (Award #1829888) to FC. and the Ratcliffe Environmental Entrepreneurial Fellowship (REEF) to DF from. We thank Tsvetan Bachvaroff for his help with bioinformatics.

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Fucich, D., Chen, F. Presence of toxin-antitoxin systems in picocyanobacteria and their ecological implications. ISME J 14, 2843–2850 (2020).

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