Abstract
After the first discovery in the 1980s in F-plasmids as a plasmid maintenance system, a myriad of toxin-antitoxin (TA) systems has been identified in bacterial chromosomes and mobile genetic elements (MGEs), including plasmids and bacteriophages. TA systems are small genetic modules that encode a toxin and its antidote and can be divided into seven types based on the nature of the antitoxin molecules and their mechanism of action to neutralise toxins. Among them, type II TA systems are widely distributed in chromosomes and plasmids and the best studied so far. Maintaining genetic material may be the major function of type II TA systems associated with MGEs, but the chromosomal TA systems contribute largely to functions associated with bacterial physiology, including the management of different stresses, virulence and pathogenesis. Due to growing interest in TA research, extensive work has been conducted in recent decades to better understand the physiological roles of these chromosomally encoded modules. However, there are still controversies about some of the functions associated with different TA systems. This review will discuss the most current findings and the bona fide functions of bacterial type II TA systems.
Highlights
Toxin-antitoxin systems are small genetic elements made up of two genes; one producing a toxin and another producing its antidote to neutralise the cognate toxin in the bacterial cell
Bacterial toxin-antitoxin systems initially thought of as mysterious selfish genetic elements are increasingly acknowledged as important genetic modules that have a significant impact on the regulation of bacterial growth, physiology, virulence and many other functions
Interest in TA research has significantly increased in the past decade, leading to discoveries of new TA systems with completely novel mechanisms of action and roles in bacterial physiology
Summary
Toxin-antitoxin systems are small genetic elements made up of two genes; one producing a toxin and another producing its antidote to neutralise the cognate toxin in the bacterial cell. The toxin is very stable, while the antitoxin is labile and degraded in the plasmid-free cells. The stable toxin exerts its toxic effects to kill or inhibit the growth of the plasmid-free cell [1]. The role of plasmid-mediated TA systems is easy to predict, i.e., the maintenance of plasmids, but the functions of chromosomal TA systems were mysterious for a long time and thought to be selfish genetic elements. Experimental evidence suggests that both chromosomal and plasmid-mediated TA systems are involved in different physiological functions of bacteria [2,3,4,8,9,10,11,12,13,14]. We bring together all recent updates about the roles of type II TAS and discuss the biological functions encoded by this important TA type
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