Novel toxin-antitoxin systems in cyanobacteria

Novel toxin-antitoxin systems in cyanobacteria

Stefan Kopfmann and Wolfgang R. Hess

University of Freiburg, Department of Biology, Schaenzlestr. 1, 79104 Freiburg

Bacterial toxin-antitoxin (TA) systems are known as genetic elements, which are encoded by plasmid as well as chromosomal loci and mediate plasmid and genomic island maintenance through post-segregational killing mechanisms. The killing mechanism is mediated by a stable toxic protein which can be inactivated by an instable protein antitoxin (type II) or an antisense RNA (type I). 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. TA systems can trigger cell cycle arrest as well as programmed cell death; accordingly, DNA replication and metabolism, translation and cell membrane organization are among their verified cellular targets. In cyanobacteria, oxygenic photosynthesis and thylakoid membranes appear as attractive additional targets, but only a single cyanobacterial TA system has been experimentally characterized so far. This is in contrast to the results of our own initial analyses, which point to a high number of TA cassettes controlled by peptide antitoxins or antisense RNA. We have experimentally characterized several novel cyanobacterial plasmid-encoded type II as well as chromosomal encoded type I systems. The expression of these toxic proteins mediated growth inhibition in E. coli, which could be abolished by the co-expression of the corresponding antitoxin. Initial results indicate RNase activity for one of the investigated toxic proteins, whose homologues in other bacteria also cause toxicity through RNase activity.