Novel Cyanobacterial Toxin-Antitoxin Systems
Final Report Abstract
Bacterial toxin-antitoxin (TA) systems are genetic elements, which are encoded by plasmid as well as chromosomal loci and mediate plasmid and genomic island maintenance through postsegregational killing mechanisms. However, TA systems exist in surprisingly high numbers in all prokaryotes and a growing number of publications 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. DNA replication and metabolism, translation and cell membrane organization are among the verified cellular targets of bacterial TA systems. In cyanobacteria, oxygenic photosynthesis and thylakoid membranes appear as attractive additional targets, but not a single cyanobacterial TA system was experimentally characterized when we started this project. In this project we worked for comparative purposes on several different cyanobacteria (Acaryochloris, Anabaena, Prochlorococcus) but then focused on the unicellular Synechocystis 6803 as was planned in the proposal. With this work we expanded the list of putative Type II TA systems in Synechocystis 6803 from 37 to 69 TA pairs and seven standalone TA components. 47 of these 69 Type II TA loci in Synechocystis 6803 are located on the chromosome and 22 are plasmid-located. With these numbers, Synechocystis 6803 is in the top-10 list of prokaryotic genomes with regard to the number of TA systems. Consequently, we can show that it is one of the most prolific sources of new information about these genetic elements. Toxins belonging to one family are not always associated with antitoxins from a single antitoxin family, as proposed in the classic classification system, but rather we observed that different types of toxins can be associated with various different antitoxins in a mix and match principle. According to the identified protein domains, 81% of all toxins in Synechocystis 6803 are predicted to exhibit RNase activity, suggesting extensive potential for toxicity-related RNA degradation but potentially also for toxin-mediated transcriptome remodeling. Indeed, we characterized in the course of this work 16 of the 69 loci biochemically or by molecular genetic approaches and found various types of RNA endonuclease activities. With the Sll1130/Ssl2245 proteins we characterized a TA system that is involved in the heat-stress induced transcriptome remodeling. Among the extrachromosomal elements, we focused on plasmid pSYSA that has the highest number of TA systems described in the literature so far and characterized several of these biochemically. Our work was echoed in the public media. Some examples: ScienceDaily: “Suicidal bacteria: Unicellular organisms occasionally poison themselves with a toxin” http://www.sciencedaily.com/releases/2013/03/130315074607.htm Der Standard (Vienna, Austria, March 17, 2013): „Warum sich das Cyanobakterium Synechocystis bisweilen selbst vergiftet“ http://derstandard.at/1362108205061/Warum-sich-das-Cyanobakterium-Synechocystis-bisweilenselbst-vergiftet Innovations Report: „Selbstmordgefährdete Bakterien“ http://www.innovations-report.de/html/berichte/biowissenschaften-chemie/selbstmordgefaehrdetebakterien-210894.html Verband Biologie, Biowissenschaften und Biomedizin Informationen: „Selbstmordgefährdete Bakterien“ https://www.vbio.de/informationen/alle_news/e17162?news_id=15711
Publications
- (2013) Toxin antitoxin systems on the large defense plasmid pSYSA of Synechocystis sp. PCC6803. J. Biol. Chem. 288, 7399-7409
Kopfmann S., Hess W.R.
(See online at https://doi.org/10.1074/jbc.M112.434100)
