Transcriptional cross-activation between toxin-antitoxin systems of Escherichia coli

Villu Kasari,Toomas Mets,Tanel Tenson,Niilo Kaldalu

doi:10.1186/1471-2180-13-45

Abstract

BackgroundBacterial toxin-antitoxin (TA) systems are formed by potent regulatory or suicide factors (toxins) and their short-lived inhibitors (antitoxins). Antitoxins are DNA-binding proteins and auto-repress transcription of TA operons. Transcription of multiple TA operons is activated in temporarily non-growing persister cells that can resist killing by antibiotics. Consequently, the antitoxin levels of persisters must have been dropped and toxins are released of inhibition.ResultsHere, we describe transcriptional cross-activation between different TA systems of Escherichia coli. We find that the chromosomal relBEF operon is activated in response to production of the toxins MazF, MqsR, HicA, and HipA. Expression of the RelE toxin in turn induces transcription of several TA operons. We show that induction of mazEF during amino acid starvation depends on relBE and does not occur in a relBEF deletion mutant. Induction of TA operons has been previously shown to depend on Lon protease which is activated by polyphospate accumulation. We show that transcriptional cross-activation occurs also in strains deficient for Lon, ClpP, and HslV proteases and polyphosphate kinase. Furthermore, we find that toxins cleave the TA mRNA in vivo, which is followed by degradation of the antitoxin-encoding fragments and selective accumulation of the toxin-encoding regions. We show that these accumulating fragments can be translated to produce more toxin.ConclusionTranscriptional activation followed by cleavage of the mRNA and disproportionate production of the toxin constitutes a possible positive feedback loop, which can fire other TA systems and cause bistable growth heterogeneity. Cross-interacting TA systems have a potential to form a complex network of mutually activating regulators in bacteria.

Highlights

Bacterial toxin-antitoxin (TA) systems are formed by potent regulatory or suicide factors and their short-lived inhibitors
Samples for RNA isolation were collected before induction (−1 min) and during a two hour timecourse post-induction (15, 60 and 120 min); mRNA of the chromosomal TA operon was analyzed by northern hybridization using DNA oligoprobes complementary to relB, relE, and relF (Figure 1; Additional file 1: Table S2)
The main finding of this study is that bacterial toxinantitoxin systems affect mutually each others’ expression and activity (Figure 7)

Summary

Introduction

Bacterial toxin-antitoxin (TA) systems are formed by potent regulatory or suicide factors (toxins) and their short-lived inhibitors (antitoxins). Bacterial toxin-antitoxin (TA) systems are complexes of a stable toxic- or growth-arresting factor and its unstable inhibitor [1,2]. They are diverse, abundant in all bacteria, except a few intracellular parasites, and are found in many archaea [3,4,5,6]. On the basis of their ubiquity and diversity, we can assume that regulation by TA must be common and beneficial in a wide range of microorganisms Their role in bacterial physiology is unclear [7,8], in part due to redundancy [9].

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Conclusion