Toxin-antitoxin (TA) systems are composed of two components. The toxin is typically a stable protein, which exhibits cytostatic or cytotoxic activity in the cells that express it. The antitoxin is either an unstable protein or RNA, which inhibits the activity of the toxin and therefore enables normal cell growth. The vapBC systems are classified as a type II TA system, meaning that both components are structurally proteins. VapC toxins contain the nuclease PIN-domain (PilT N-terminal) and act by inhibiting cellular translation. Antitoxins VapB can be structurally variable, however they all act via binding to the toxin and forming a non-toxic complex.
Microcystis aeruginosa is a bloom-forming cyanobacterium containing the highest number of TA loci of all bacteria, according to the TADB2.0 database. Despite this, TA systems of M. aeruginosa remain poorly understood. In this work, we have performed a bioinformatics analysis, using TAfinder and TASer to find new putative TA loci in the genome of M. aeruginosa PCC 7806. We have identified 370 potential TA loci, of which 57 are vapBC systems.
From our list we selected six TA pairs and successfully constructed cloning and expression vectors containing the genes for either the toxin or the toxin-antitoxin pair. Three toxins, VapC29, VapC33 and VapC45, were also experimentally characterized by assessing their effect on the growth of Escherichia coli cells during expression. Based on our results, VapC33 likely possesses no toxic activity, while VapC29 and VapC45 could potentially act as active toxins in vivo.
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