Abstract
Bacterial toxin–antitoxin (TA) systems are genetic elements, which are encoded by plasmid as well as chromosomal loci. They mediate plasmid and genomic island maintenance through post-segregational killing mechanisms but may also have milder effects, acting as mobile stress response systems that help certain cells of a population in persisting adverse growth conditions. Very few cyanobacterial TA system have been characterized thus far. In this work, we focus on the cyanobacterium Synechocystis 6803, a widely used model organism. We expand the number of putative Type II TA systems from 36 to 69 plus seven stand-alone components. Forty-seven TA pairs are located on the chromosome and 22 are plasmid-located. Different types of toxins are associated with various antitoxins in a mix and match principle. According to protein domains and experimental data, 81% of all toxins in Synechocystis 6803 likely exhibit RNase activity, suggesting extensive potential for toxicity-related RNA degradation and toxin-mediated transcriptome remodeling. Of particular interest is the Ssr8013–Slr8014 system encoded on plasmid pSYSG, which is part of a larger defense island or the pSYSX system Slr6056–Slr6057, which is linked to a bacterial ubiquitin-like system. Consequently, Synechocystis 6803 is one of the most prolific sources of new information about these genetic elements.
Highlights
Toxin–antitoxin (TA) systems are small genetic elements composed of a stable toxic protein and its unstable cognate antitoxin
To identify undetected type II TA loci in Synechocystis 6803, we examined its genome and plasmids using the RASTA system [24], leading to the identification of seven new TA loci, which were not addressed in the TA database [17]
We retrieved the gene IDs of Synechocystis 6803 protein sequences containing these domains via the Pfam database [27] and searched in a case-by-case analysis the genomic neighborhood of the predicted toxins or antitoxins for potential cognate TA
Summary
Toxin–antitoxin (TA) systems are small genetic elements composed of a stable toxic protein and its unstable cognate antitoxin. They are encoded on chromosomes as well as on episomal genetic elements and are widely distributed throughout the prokaryotic domain of life. Many functions have been assigned to TA modules, ranging from plasmid maintenance to persister cell formation and stress response (for reviews, see [1,2,3,4,5,6,7,8]). TA systems exist in surprisingly high numbers in all prokaryotes and a growing number of studies suggest TA systems with milder effects to act as mobile stress response systems which help certain cells of a population in persisting adverse growth conditions. Whereas the toxin is always a protein, both proteins as well as RNA molecules have been found as antitoxins
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